Syncronizing other folders than \My Dropbox is’n’t possible in current Dropbox client version. However by create a link to a folder in the My Dropbox folder you can sync any other folder to!
When you’re on Windows XP use Sysinternals Junction to create a link.
When your on Vista, Server 2008 or Windows 7, use the built in MKLINK command.
My situation was to link My Documents folder, which is redirected by a Group Policy, to \My Dropbox folder. On my client machine I installed Dropbox, configured My Dropbox folder to X:\My Dropbox. Next I had to make a link in X:\My Dropbox\Docs to My Documents folder:
mklink /D "X:\My Dropbox\Docs" "\\anatool\home$\username\My Documents"