Finally Dropbox has arrived with a solution which many wanted, ability not to allow some of the folders ( and sub folders ) not to sync but still keep it on my Dropbox account online. Why ? I might be running out of space on my hard drive.
After installing the build, go to Preferences > Advance Tab > Click on Selective Sync and Switch to advance mode for selecting / deselecting particular folder. According to this post on Dropbox Forum, this is a experimental release which you can give a try.