Originally Posted by shayaknyc
The operating system, and more importantly, Dropbox, thinks that the folder actually exists in the Dropbox folder. Any time you add or change anything using Calibre, the files and metadata will be automatically synced with your Dropbox account.
Yes, this is very handy, I use the same technique for syncing my library between Linux and Windows.
One note: as per the recommendations of a Dropbox developer* you should have the "real" files residing within the Dropbox folder, and make a link from your "%userprofile%\Calibre Library\" to that instead. Calibre (and most other local software) won't care, but Dropbox uses some extremely complicated code to be able to handle change detection and file handling across different combinations of OS and filesystems, it might get confused if you trick it with links.
* I saw this in a discussion about storing live save game files in Dropbox. Regrettably I can't find it now, but the explanation from the developer was compelling.
Edit: It seems that this small utility works this way, it has a simple gui: http://dropboxifier.codeplex.com/