Doesn't there have to be some evidence? Simply speaking to a player is not proof of tampering, there would need to be somebody going on the record as saying there was discussion of Favre going to the Vikings. I doubt the Minnesota coach will admit to that (perhaps his recollection gets fuzzy), and I doubt Favre would say anything either, as it would help the Packers HC and GM - who are obviously not on the top of his Christmas card list right now.
On the other hand, Goodell has spoken earlier this year about cracking down on teams that are guilty of tampering, and also about needing only a 'preponderance of evidence' as a threshold for imposing punishment.
If nothing else, the NFL has succeeded in keeping themselves in the news and becoming a sport to follow twelve months a year.