According to their forum, DropBox plays like a feisty five-year-old in a sandbox, i.e. not so well:
Dropbox doesn't play well with
FileVault: Spotlight won't find files
in your FileVault locker including
your Dropbox, if it's in the locker.
Solution: Move your Dropbox files
outside of FileVault. They won't be
encrypted (and you can't move them
back in), but it's a temporary fix.
Alternative solution: You can teach
Spotlight to index all files in your
locker by adding your home folder to
the list of files Spotlight should be
prevented to index and subsequently
removing it again. You can find the
list under "Privacy" in the Sportlight
settings in System Preferences. Once
you added and removed the folder
again, Spotlight should start
Dropbox doesn't sync until restarted
for some users: No solution yet.