I caught my MacBook's fan noise, so i looked to see what was sucking CPU.
It wasn't, surprisingly, Flash. The MacBook was trying to index the backup drive attached to my Time Capsule upstairs, even as it did its routine Time Machine backup to the remote image.
I'd seen that on other machines, so I knew the fix. I added the remote mounted backup drive to my Spotlight Preferences Privacy tab. The fan spun down a few minutes later.
It's a surprising behavior. Does Spotlight also try to index a TM backup on a physically attached drive? Obviously this kind of indexing is pointless, just a waste of bandwidth, storage, CPU and power. I'm surprised it's not been fixed.
Update 6/22/11: There are odd consequences with doing this, esp. in 10.5. This 2008 discussion is interesting -- TUAW Tip: exclude your Time Machine disk from Spotlight indexing and 10.5: Spotlight and the Time Machine disk - Mac OS X Hints