I just committed a change to svn to the way debug.log is written that means you need to change your logrotate configuration.
Why the change: before, bitcoin was opening and closing debug.log on every print. That caused it to be unusable on Macs running FileVault (and slower than it aught to be on various other systems).
After subversion rev 183, bitcoin will keep debug.log open and just append to it.
If you're rotating logs using the unix logrotate command, just add 'copytruncate' to its configuration file and the right thing will happen. I changed the example in my post above.