There's already at least one Portal cartoon in production. That one, however, looks like something Dreamworks would come up with. If you'd rather a Portal cartoon look more like something Ren & Stimpy creator John Kricfalusi would dream up, though, you might want to take a look at these.
These pieces are the work of artist Sandra Rivas, whose style is heavily influenced by John K.. What's cool is that she's turned all the cores and computers into people, and in doing so, has absolutely nailed them. Especially Wheatley.