If I switch on FileVault 2 in Lion, can I still access my OS X volume from Boot Camp (Windows 7)? Are both volumes encrypted and decrypted together, or does it only affect the OS X volume?
And the correct answer is: No.
Encryption only affects the OS X volume. Booting into Boot Camp, the necessary password prompt to allow decryption is bypassed entirely. The volume becomes an opaque blob to anything but a running OS X system.
In short, yes. John Siracusa from Ars Technica has a very long review of Lion with much more detail. Also, on a recent podcast he said to think of File Vault 2 this way - once you turn on your computer and enter the password (before anything boots) the disk is essentially unencrypted. Time Machine backups, Boot Camp, network access, etc all happens (essentially) unencrypted.
While I haven't tried your particular method myself I have FileVault running and I can access my system just fine in other ways. I hope this helps!