Season 6 has been announced for release on May 8. http://tvshowsondvd.com/newsitem.cfm?NewsID=6897 This was a great season too; I like that all the episode titles were titles of songs by the Who.
Season 4 came out May 9, so it hasn't quite been a few months. I expect we will see Season 5 sometime in November or so, which has been the pattern for a couple of years now. I don't think the show really fell off in quality. I think it had great stuff when Eric and Donna got back together...