It seems the tech preview is indexing forums with session IDs. I have one on phpBB, no mods installed, with pages showing in the site: command. Google shows this same forum with only the index page. Nice
I think most of the phpBB forums are already optimized for the spiders. I'm just too lazy to do the mods But it seems, with regards to MSN, there will be no need for it. Let's just hope G follows suit.