There are talks that the switch to PoS could happen this summer but is it too early, or the date is real so should miners already acknowledge that soon enough they will not be contributing as much?
The switch to the Casper Proof of Stake protocol will happen in the Serenity release, and the latest information is from March 5 2016:
The protocol decisions made here are by no means final; many of them are still actively being debated within the research channels. The next few rounds of PoC releases will thus move toward creating something resembling a Serenity node implementation, alongside a proper p2p networking layer, with the eventual goal of running a Serenity testnet between multiple computers; at the same time, our research team will continue hammering away at the finer details of the protocol and make sure that every single protocol decision is made correctly and well justified.
Additionally, we will be coming out with more accessible materials on the Casper protocol specification and design rationale in the next few weeks, covering both the broad consensus-by-bet concept as well as specific design decisions ranging from validator induction rules to betting mechanisms and block proposer selection.
I would use PoW checkpoint blocks to prevent certain PoS attacks. Every N PoS blocks, a PoW block would need to be be mined (and can be mined at any time before then, with less reward). PoS chains that had no checkpoints within a certain height would be invalid, enough to prevent long PoS forks. PoW blocks do not contain transactions and are linked to each other, as well as to the main PoS chain.
No way to double-spend attack the PoW system, because no txs. No way to double-spend attack PoS with a very long-fork, without also expensively attacking the PoW system. So the real PoS issue of a system being offline for N months, and then getting exposed to malicious nodes that trick it into accepting a bad chain cannot occur without also PoW mining that bad chain.
Just decided to chime in as ether is one of the crypto currencies I mine. I've been researching as much as I can on this change. The date of the change is of less concern for me as is the profitability. For example ethereums global computer relies on the current computing power of all its miners to be able to execute smart contracts and decentralized block chain based applications. From my research it appears that this compute power would shift from machines designed for heavy computational use aka mining rigs to the nodes themselves. The theory in pos if I understand it correctly is that they will remove the problem solving portion of the algorythm so individual nodes with marginal compute power will participate. However their seems to be a great error in this concept. Given the compute power of a mining rig (Typically around 30 Tflops per machine) even with the pow taken out if miners exit the network for alternative pow currencies then the network would loose 99% of its compute power. This would mean that the network would not be able to live up to its goals and we could easily see a rapid abandonment from ethereum platform development and rapid network degradation until only a few large companies are controlling the whole of the network thereby making it centralized. Software giant Microsoft has already shown interest in ethereum so it's likely they would end up being the company running most of the nodes thereby making etheteum no longer an open source decentralized system. Microsoft or other un named companies could then quickly fork the whole network and close source the new fork thereby taking over. Pos looks good on paper but it's a flawed solution. A better solution would be to figure out a way to keep your miners actively participating in the block chain so that all that compute power can be fully unleashed because with the pow the compute power is essentially governed to ensure 1 block every 14 seconds. As the network becomes more popular for decentralized applications (which have amazing possibilities like running a virtual server on a block chain as future revisions to the programming language occure) the difficulty of the problem solving is adjusted based on current network demand to ensure high availability and rapid transaction execution. To avoid possible network attacks from a financial standpoint once the network is being utilized by enough users for content instead of just currency you begin an accelerated process of doubling block rewards instead of halving them with the goal of having gas only incentives as quickly as possible. Once the ether is fully mined and in circulation the difficulty of the pow could be decreased to the lowest possible number thereby giving birth to the most powerfull computer mankind has ever concieved of. Imagine AI being able to utilize it. Storage for the masses. With my solution ethereum could become not just bitcoin 2.0 but cloud computing 2.0. And decentralized virtual machines 1.0 where the machine can not possibly be taken down or tampered with. However it requires the miners to remain in the network because essentially they are the network. And as for carbon emissions mining rigs are carefully built to take into account as low of energy consumption as possible and the best performance per watt possible. The innovation of miners like myself can far exceed that of large companies. On a shoe string budget over the years we've come up with solutions to many problems. We have developed tech that people don't even realize exhist unless they're actively mining and to waste this by switching to pos would be 3 steps backwards and be the downfall of ethereum. Casper would die for a second time (pun intended). I end this post with the beginning... The Times 03/Jan/2009 Chancellor on brink of second bailout for banks.
In summary their is no definitive time line for the pos fork. However we just started the homestead release and as you can see from my concerns and suggestions above a lot of issues related to this have to be decided. We could see many other revisions before a switch to pos and there is no evidence that pos will ever roll out. It is part of the publicly released time line but in that time line it explicitly states that it is subject to change and their may be more sub revisions added. So keep mining my friend and relax. Check officiall documentation regularly for announcements. I'm certain they will give us plenty of time to figure out the best way to transition or if we even will transition.