Same problem. When working with or moving files in folders other than the dropbox folder, I still find the dropbox application taking up large amounts of CPU. Dropbox also, in the last few months, has been messing up my spotlight database and causing mdworker and fontd to use all available cpu.

Then I start getting console messages like this: mds (Error) FMW: WE ARE DROPPING FMW EVENTS!

All disappears after disabling Dropbox (and rebuilding caches and the spotlight database)

Behaviour seen on multiple macs I own.