If true, I think Seau will have a bigger impact than Colvin.
While Colvin is filling a real position of need, he was clearly in decline before his injury last year. He struggled setting the edge, and showed less burst as a pass rusher. He did total four sacks through 11 games, but remember that that was with 20 and 30-point leads through those first 11 games, and that put him on pace for just six for a full season.
Seau was the Pats best ILB last season. He was a great tackler, diagnosed plays instantly, held his area of containment, blitzed, and dropped into coverage effectively. He had four great passes defensed, and three INT's - the best inside coverage on the team from the oldest player.
Matching Seau with Mayo could be very effective, and enable Bruschi to rest a lot more.
Lining up Vrabel-Seau-Bruschi-Colvin probably offers the most experienced LB corps in the league, but that's not a happy thought.
On a potentially related note, I have noticed that Redd is injured after tallying a couple special teams tackles. If he is the odd man out, I guess it doesn't make much impact, but it's mildly disappointing if that's all he makes of a great opportunity.