While I was setting up my new Mac and futzing about with it in general, I stuck in a USB flash drive that had been formatted with ext3. Needless to say it didn’t mount it or anything, and I removed it shortly after. I then noticed that if I moused over the notifier area in the upper right corner of the screen (where the date, volume, Time Machine, etc. controls are), the Spinning Rainbow Ball of Doom showed up. Everything else was fine though.
I googled around and found the solution in the last comment on this MacRumors thread.
Open Terminal and type killall SystemUIServer, and you’ll be all set.