Now, it's not official, so take it with a grain of salt, but one online venue called CinemaSpy is reporting this as a done deal.
This claim is bolstered by the fact that Smallvile has been building new sets (supposedly) and Supernatural's creator, Eric Kripke, has been talking about potential storylines for Season 6. Plus Dawn O., who has a history of not really caring for either show, has gone on the record recently with positive statements on both shows.
Also in the shows' favor is Supernatural recent win at earning a People's Choice Award for Best Scifi/Fantasy Show and Smallville's continued success on the CW in the death zone of Friday night.
I'll wait for an official release. There are always unexpected factors that can cause a show to get canned, even when everything seems fine. Though, at this point, it seems more likely than not.
And meg_tdj ? They are also reporting Jason Momoa got the Conan gig ;)
ETA: CW has slammed down the reports of renewals. Told you to take it with a grain of salt ;) But I think they have a good chance of renewal anyway.