The release date was revised for May 2009, at least, in the current roadmap thread for 2009. We've moved the date to June, that's for v3.2. There's no multi-column combo box feature in there yet, though. The feature didn't make it into the official list. I think it is a candidate for a release later this year, so we'll post a new roadmap update in June with the release of v3.2 and you should see if it will be in a release this year.
Odi Kosmatos
VP, R&D, Xceed