There is a fairly significant bug when using FileVault in Leopard that as of OS 10.5.2 is still not fixed. Basically, when using FileVault, the LaunchServices database is not read properly when you login to your FileVault account after a system restart (not just a logout/login — a full restart).
The reasons why it doesn't work are a little shaky, but basically, OS X fails to read the LaunchServices database and simply reverts to default settings. This means that any changes you make to the default applications -- changing your default web browser, RSS reader, FTP client, or changing which application files of a specific type are opened in -- will all be reversed as soon as you restart. The changes are, in fact, still present in the database, but they are not being read by the system.
Now, obviously, Apple needs to fix this ASAP, and I encourage everyone to submit a bug report on it. Until this happens, however, there is a fairly simple fix to force the database to be read on login.
To fix launch services, enter the following command in the Terminal:
Mac OS X Hints
http://hints.macworld.com/article.php?story=2008031217120475