Zotero has just released its public beta of Zotero Sync, allowing you to maintain your Zotero library on multiple computers. This is a feature I've really been waiting for, and it's a precursor to the forthcoming Zotero Server providing online storage of references.
Read the documentation at that link -- there are warnings about changes to library formats and such that are worth paying attention to before proceeding.
I haven't tried it out myself yet, but will post my impressions after I've worked with it for a bit.
Small update: I got an error message when I tried to update my database to the new version. I worked around it by exporting my library, deleting everything in my Zotero directory (after zipping up a backup copy, of course), restarting Firefox and importing my library again.