We aren't hiring beta testers for PV13 at the moment (it's still a little too early), but when we do, we'll usually hire local testers for internal playtesting to start, then do a closed beta (Friends and Family, first), then do an open beta.
For the other projects, I'm not sure -- it's being handled on a project by project basis.
You should check out:
http://www.gamasutra.com/
That's a good place to start looking.
pax,
-Chris