What The Folk! is on Twitter. I've been using it personally since it started almost. A few years now iirc. I signed up a WTF! account a couple of years ago but have really only bothered with it in more recent times for the site. If you visit the main site I do plug it there http://twitter.com/whatthefolk
And no, neither Bret or Jemaine are on Twitter. They are fakes. Both Jemaine accounts
No Rhys either. He doesn't have time for it. Too busy. No Kristen.
Arj Barker is real, as is Todd Barry, Eugene Mirman. O Rob Brydon is there. HardnPhirm. Aziz Ansari. As are some of the Conchords real life friends who also happen to be comedians, actors and musicians.
You can follow @valebrity to verify which celebs are real and fake. They usually track down and post on who is who.
I should mention that the only public point of contact they maintain for fans is their MySpace page. If either of them start using Twitter (more likely to be Jemaine than Bret as he really doesn't do the whole computer/internet thing) I shall let people know.
*edit* Removed Demetri Martin as is a fake I just read. Apparently *sigh*