I usually develop in my local Dropbox folder. Some files in the tmp-folder get locked by the browsers (and keep Dropbox busy), Growl throws exceptions and so on.
Th
You can exclude a directory from being synced in Dropbox by using Selective Sync: http://www.dropbox.com/help/175/en
Basically select the Dropbox preferences and go in Advanced. Select then Selective Sync and look for the folder that you want to exclude from syncing [there is also an advanced view if you have to go in a deeper than 1st level dir depth]