Ok, so the new beta version of Zotero has been out for about a week. In every respect the results are excellent. With a large database (I'm using one with about 5000 items) the new version (1.5) is significantly faster than 1.06 at the basic tasks of adding tags and updating lists. There are more citation styles to play with (although that's never been a major issue for me). On a smaller database (one with hundreds rather than thousands of documents) the new sync-ing function works very nicely. Now you can safely store and update your database using their server.
Some more things that I'm waiting for from Zotero:
1) hierarchical content structures. This would allow you to link -- for example -- the letters that you find in a specific file in the archive to each other and to the actual file. It would also open the way towards content types like 'actors' that would allow you to begin to actually structure the records like real life.
2) integration with are National Archives web site. Alas that will require an entirely new kind of document type called something like 'government correspondence'.
3) collection sharing. This will happen soon, and I think that it's going to have really powerful effects on the way we teach post-graduates and undertake collaborative work. It's also going to open a can of worms about property rights in data that have not yet really been explored in the universities properly ...