While no one but Favre himself can state his reasoning for turning down the Packers ad deal, I will attempt to put myself in his shoes in order to conjecture as to why he refused the $20 million.
Reason number one: he's rich enough already! His 2007 base salary was around $11 million (WikiAnswers estimate). He could make more in two years of playing than in ten years of marketing.
Reason number two: he hasn't exactly been buddy-buddy with the Packers franchise this last month or so. Why would he want their blood money? Taking that deal would not only help them out with their QB mess, but it would leave him advertising for them.
Reason number three: he loves football. This reason is probably the one that hits closest to home. The financial world would wonder how sane any guy is that could turn down $20 million in sure money. The sports world knows better. We know what it's like to love the game. We know that when you give your life to playing football, it never leaves you. Regardless of his March press conference, Brett Favre still wanted to play football. Now, he will. We're just waiting to see where.