We just had a vote, and Tapestry 5.1 (that is, release 220.127.116.11) is a beta release. At this point, everything is stable except for minor changes to the brandy-new URL rewriting APIs. I've been chasing down bugs and improvements for the 18.104.22.168 release, which will probably be created and voted on later this week. My guess is that 22.214.171.124 will be the final/stable release for 5.1.
I'm already thinking for 5.2. Ditch Maven for Buildr? Strong possibility. Start using Groovy for tests? Why not? Finally get Spring Web Flow integration working ... you betcha! Portlets? Shouldn't be a problem. Release date? I think before the end of the year.