I wonder if the story that Favre's agent is looking for a team to trade for him was held back a couple of days so it wouldn't be confused with an April Fools Day joke. But as Tom Silverstein said, he's a Hall of Fame quarterback who can still play, and it was only a matter of time before these rumors started.
There could be something to the rumor. One of the things that has exhausted him over the past few seasons is GM Ted Thompson's apparent distaste to add, or keep, veterans on offense. He's kept RT Mark Tauscher and WR Donald Driver happy, but made Favre consider pushing for a trade after Thompson only half-heartedly pursued a trade with Oakland for WR Randy Moss. Also, Favre has been known to change his mind back and forth on retirement over the past couple years, so there's no reason he hasn't done it again.
But what really makes me believe that it's false is that it didn't come from Favre. He wasn't shy about his frustration over WR Randy Moss. It seems unlikely he would have said all that he did after his retirement was announced about how the 2007 season really wore him out without meaning it.
We shall see.