While I have seen much discussion about FileVault and its disk images on the internet and here on macosxhints, I was left underwhelmed by everything I saw. It is not too hard to mount a FileVault disk image, but all of the hints I saw exhibited two problems:
hdiutil attach /Users/username.sparseimage -stdinpass -owners on \
-mountpoint /Users/username -nobrowse
The -owners on parameter means permissions on the disk image will be respected, and only the user with access to that disk image will be able to access its contents. The -nobrowse parameter means the image is not recognised by Mac OS X as a device, and is therefore not mounted in the Finder for everyone logged in. It is still usable for anyone who has the necessary permissions to view its contents.
Mac OS X Hints
http://hints.macworld.com/article.php?story=200512151629177