Will Bitcoin be linked to Ethereum in the future? If so - how will it link, and when is this planned to happen?
In the near future, it will be possible to verify transactions made in Bitcoin, in Ethereum, using BTC Relay (http://btcrelay.org/, https://medium.com/@ConsenSys/taking-stock-bitcoin-and-ethereum-4382f0a2f17#.v4ghgl4uk).
Thus, at the very least one would be able to build a decentralized exchange between Bitcoin & Ether. Using this transaction verification scheme, it will also be possible to build a federated multi-sig scheme in Bitcoin, that allows the creation of Bitcoin-as-a-token on Ethereum. Once you don't want to use that token anymore, you can use this federated multi-sig scheme to release the Bitcoin again on that side.
If Bitcoin implements the possibility to verify SPV proofs, then it would become possible to implement fully trust-less transfer of Bitcoin over to Ethereum.