Disclaimer: initially, I was reluctant to pass this one on, but it looks like it may be more credible and less speculative than I first thought.
During IFA today, Samsung confirmed that Gingerbread is next and will be Android 3.0, and that it will be succeeded by Honeycomb (3.5). It is possible that things will change between now and then, but this is an official, public word from Samsung in direct response to a question - fairly concrete.
They also divulged that they're working on a larger (presumably larger than the Tab) tablet for Honeycomb. As for the Tab itself, it'll ship with Froyo (which we already knew), and will be updated to Gingerbread once it's released.