He should have retired before last year - and I'm not just saying that because I'm a Vikes fan (I must also confess I am originally a Chicagoan, so I REALLY hate the Packers coming from both sides of Cheesehead-land). I'd say that even if he played for us.
He made a lot of bad mistakes last year that can't entirely be blamed on the lousey line or his less-than-stellar receivers. He did have quite a disadvantage with last year's team, and he tried hard - too hard most of the time - to make up for it, which got him into trouble. But he made mistakes that a QB of his caliber should not be making.
I really am surprised that he's coming back this year. I thought for sure that when Sherman left, that was the end of Favre too, as he had said before that he didn't want to have to learn a whole new system. Now he'll have to, and it will be with a team that didn't improve much in the off season from last year's squad.
Honestly, I think he's being selfish. He should have let the Packers begin life without him, as they really are a team that needs to rebuild. It's time to start preparing the guy they drafted last year (what's his name?) to take over the team, again, something that should have happened last year.