My reckoning so far is this (this isn't gospel though, some driver placings are an educated guess):
Ferrari - both drivers confirmed as staying put
McLaren - both drivers confirmed as staying put
Renault - i reckon Alonso will stay, the second driver seat is still pretty open though... Di Grassi and Grosjean are probably favourites right now.
BMW - both drivers confirmed as staying put
Toyota - both drivers confirmed as staying put
STR - hard to say. Bourdias, Sato and Piquet all look like strong candidates for a race seat though.
Red Bull - Webber and Vettel - both confirmed
Williams - both drivers confirmed as staying put
Honda - i think it'll be Button and either Barichello or Senna
Force India - i think both drivers are staying, can't remember if they've been confirmed or not
EDIT: just found this page, would've saved me typing all this up!!
http://www.formula1.com/news/features/2008/10/8584.html