Matt Birk is no longer a Viking (SI, Strib).
Five years ago Matt Birk was an incredible center. When I would watch replays of successful running plays, very frequently there was Matt Birk pulling and leading with terrific blocking. In recent years I've found him much less noticeable. That may not be because Birk's skills diminished, but that perhaps Brad Childress' version of the West Coast offense doesn't call for a center to be out in the open field making noticeable plays. Either way, I don't know that the Vikes will miss Birk's blocking that much at this point.
What they will miss, however, is his intelligence and leadership. When Matt Birk was injured in 2005, the Viking offensive line became...well, terrible, at least for the first half of the season. He seemed at the time to be the mind holding the Viking offensive line together. Whoever replaces him at center (Ryan Cook?) may make the blocks, but I doubt he'll be able to do all the things Matt Birk does.
Birk will be 33 in the fall; he should have good football games left to play. It's not entirely unreasonable that the Vikings decided to let him go, but it wouldn't have been entirely unreasonable for them to resign him either.
UPDATE: read about potential replacement centers John Sullivan and Anthony Herrera at the Star Tribune.