Unfortunately, I have no source to link, but it's been said that Brett was "offended" by the Vikings offer of more money, implying that's why he'd come back. Apparently Brett just wanted to retire and walk (limp) away.
I don't know how true that is, but after hearing that and watching the video of Ed Werder (sp) talking to him while he's in his truck ... kind of made sense to me.
I don't think Brett wants to come back an risk going 2 - 2 against the Packers or getting hurt worse than he did against the Saints. There's also a few dubious records he's within reach of that I don't feel he wants any part of. I also think going out with the statistical season he had gives him solace that he went out on 'top' too.
I'm confident he'll be back, but I can see why he would call it quits too.