How does Apple's Power Nap interact with File Vault?
In general, Apple disables FireWire DMA in states where the computer isn't unlocked for use. There were some states they missed early on (see Todd Garrison's description of the problem), but they tightened the access controls in 10.7.2 (see CVE-2011-3215 here).

Power nap adds a new DMA-should-be-disabled state, so there's a possibility that it might have reintroduced this vulnerability, but I would expect Apple to have handled this correctly. Unfortunately, I don't know of anyone who's actually tested it (and I don't have a power-nap-capable Mac myself).