Brett Favre’s career may be winding down, but until the NFL season starts and his name is not on a roster, it is not over. John Elway may have been the “King of the Comeback,” but Favre will always be “Keen on the Comeback.”

After a tearful retirement from the Green Bay Packers in March 2008, Favre changed his mind and returned to the NFL. His one season with the New York Jets was forgettable, and three months ago he informed the Jets he was retired. Again.

The Jets released Favre earlier this month, paving the way for the three-time MVP to sign with any team.

Enter the Minnesota Vikings.

“He’s a Hall of Fame quarterback. He’s a great competitor,” Viking president Mark Wilf told The Associated Press. “Ultimately, you’ll have to ask Brett what his plans are, but, sure, there’s interest in Brett Favre. But again, it’s part of a process we have in general with any of our players. We’re always looking to make our team better.”

Still his flirtation — a two-day dance, mind you, since I had to look up who started most of Minnesota’s games at quarterback last season — with the Vikings leaves questions that forge only two plausible answers.

First, athletes always have a difficult time saying goodbye. Second, productive NFL quarterbacks are hard to find.

With the exception of Barry Sanders, who left the Detroit Lions after 10 seasons never to be heard from again on his way to the Hall of Fame, few leading players can leave the stage and stay gone. Favre is hardly alone here.

As we contemplate his return at the tender age of 39, let’s remember that Rickey Henderson was boucning around in the minors at 46, Michael Jordan was a 40-year-old second option for the Wizards earlier this decade, and Lance Armstrong is checking his spokes somewhere this morning trying to reclaim his glory days gone by.

Roger Clemens even may be contacting his trainer. Wait, bad example.

Still, athletes are notorious about hanging on rather than moving on once and for all.

“Part of the process of being great is having an obsession,” baseball Hall of Fame pitcher Jim Palmer told McClatchy News Service. “That’s the way great athletes are able to have Hall of Fame careers. So there’s this drive that’s hard to give up. But eventually we all have to.”

Palmer should know. The former Orioles Cy Young Award winner lasted two innings in his return to baseball at 46 — six years after his final big-league season and less than a year after he was inducted in Cooperstown. Plus, the man who was almost as famous for his Jockeys as his fastball ignored his own advice.

As legend has it, Reggie Jackson once asked Palmer when the slugger would know to call it quits.

“The pitchers will tell you when it’s time,” was Palmer’s response.

The NFL in general, and the Vikings in particular, can’t make the same claim. Roughly half the NFL has a solid answer at quarterback, and a handful of those teams are counting on guys who are one bad season away from being forgettable flashes-in-the-pan. Seriously, if Joe Flacco throws 20-plus interceptions this season, are you going to be shocked when the Baltimore Ravens are looking for alternatives?

Favre the hot-shot gunslinger has long since been a memory. His days of leading the NFL in passing yards or touchdowns are forever done, gone the way of blue-chip bank stocks.

Still, this is the league that has made a world of hype for Mark Sanchez, he of the one solid starting season at Southern Cal, and a trio of Matts — Stafford, Ryan and Cassel — who collectively hold the future of three franchises in their still-inexperienced right hands.

There is obviously opportunity across the league — yes, Gus Frerotte started 11 games for Minnesota last year and is listed as the starter heading into the offseason workouts on the team’s Web site. He joins a healthy and indistinguished list of possible opening-day starters such as Sage Rosenfels, Kyle Orton and Brian Griese.

And as long as there are teams in need — and aging athletes starving for that one final fling — there always will be the less-than-triumphant return. And those awful images of Jerry Rice’s No. 80 Raiders jersey or Emmitt Smith’s No. 22 in Cardinals red and white.