Because in this day and age no TV show ever stays in a timeslot long, the WB has announced that Supernatural will be moving to Thursdays at 9 pm starting March 16. This pairs it with Smallville, which has garnered surprisingly good ratings this season.
As this announcement comes after the UPN/WB merger news, I would guess that this is a tryout with the blessing of the new network, and that if Sam and Dean can maintain the audience their fellow Kansan is getting, we will see the two shows together on Thursdays next fall as well.
This also clears the way for a possible Tuesday night pairing of Gilmore Girls and Veronica Mars on Tuesdays after the merger.
In any case, remember to reset your TiVos and VCRs before March 16.