If Tor or I2P support is not yet practical, is there a guide of best practices to help protect IP and other metadata?
If Tor or I2P support is not yet practical...
Running over Tor currently won't work due to Ethereum's requirement of UDP port 30301, and the fact that Tor only supports TCP.
I2P is actually more interesting. There are two protocols implemented on I2P: NTCP (NIO-TCP), and SSU (Secure Semireliable UDP). It'd be an interesting project to see if Ethereum could be made to work using these.
With regards to what protection is present, the main thing to note would be that all communication between nodes is via the ÐΞVp2p Wire Protocol (an encrypted and authenticated transport protocol), as discussed in this previous thread.