Update (2008-04-24): I managed to get rid of the xmlns-related errors (.replace() to the rescue ;), so the extension now accepts markup that follows the latest RDFa DTD (including @typeof). And while at it, I created versions for win and mac.
One of the reasons I haven't been using RDFa in production is the problem of quality assurance (a.k.a. plain old html validation). Not because RDFa isn't valid markup as such, but the main tool I'm using during development is Marc Gueury's excellent HTML Validator Extension for Firefox. RDFa is valid XHTML+RDFa, but XHTML+RDFa is not HTML, so the extension reports dozens of errors starting with the unrecognized Doctype declaration. The W3C Markup Validator supports RDFa, but I often develop while I'm offline, or on a non-public Web server, and the little "0 errors / 0 warnings" message in the status bar is more convenient than having to send markup to an online service.

Yesterday, however, I started working on an RDFa generator for one of Intellidimension's projects (Very interesting to see them use RDF big time, while many of us are still experimenting and thinking about potential markets, BTW). So, now that the RDFa-caused messages made it almost impossible to spot real HTML errors, I wondered if the add-on could perhaps be hacked to accept RDFa as well. Long story short: It can, to a certain extent. I don't know if arbitrary XML namespace prefixes (xmlns:foo="...") can be supported by a pure DTD/SGML-based validator (the FF extension uses openSP). FWIW, I couldn't get it to work.

Apart from that, RDFa-enabling the extension was mainly copying the RDFa DTD and a set of modules to the plug-in's SGML library. It now happily accepts RDFa attributes (about, resource, property, datatype, content, etc) and makes my life a little bit easier. If anyone has an idea how I could make it accept (non-predefined) namespace prefixes as well, I'd appreciate hints.

The tweaked extension is so far just a hack. I didn't even ping Marc yet or change the internal ID, so any extension update will remove the RDFa functionality. You can try/download it if you like (windows version), but I may have to take it offline should Marc not be happy about the re-distribution.