There's a certain amount of spookiness that goes on regarding Apple's FileVault encryption. As everyone knows, it's very difficult to get into anything that's encrypted without the proper keys. But let's assume for a minute that you already have the key via, say, a court order for a suspect in a law enforcement action. Even with the key, however, there's still a lot of ambiguity about just how well the suspect's data (and in particular, deleted data) may be protected.
I've written a brief titled FileVault Imaging: Apple's Dirty Little Secrets that explains exactly how you can mount a FileVault volume (with the key, of course) to obtain the raw disk image. I will also illustrate how deleted data can be preserved inside a FileVault, and how a free space wipe does nothing to purge deleted data from inside one. The protection offered by an encrypted volume seems to aggravate other security issues, possibly making deleted data less secure.
[robg adds: The referenced brief is not about breaking into FileVault volumes, but about how data is stored and managed within those volumes -- in particular, deleted data. The section on the free space wipe was the most interesting to me -- this feature basically doesn't work as intended if you're using FileVault. I haven't tried to confirm that finding for myself, but the provided example seems fairly clear-cut in its proof.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20081011065541585