After some recent Dropbox problems I've been having (not the first unfortunately) I added an entry on the Dropbox wiki explaining what happens if you are so impertinent as to add directories containing symbolic links to your Dropbox folder:
[UPDATE - dropbox took down their wiki, but I reposted the page here.]
One day maybe we'll have something with the correctness and robustness of Unison and the convenience of Dropbox. People will dance in the streets. I think Dropbox can be that solution if they work at it.
UPDATE - these issues are explained at greater length in this post: