Even more surprisingly, the compensation for Seymour, drafted in 2001, and now on the last year of his contract, is a first-round draft pick.
But, according to Adam Schefter that draft pick is in 2011, not 2010.
It's not clear whether this move has been in the works for a while, or if this was something that happened "out of the blue." But it's clear that with three key players for the Patriots—Seymour, defensive tackle Vince Wilfork, and guard Logan Mankins—all set to be free agents in 2010, that it was unlikely the Patriots would be able to keep all three of them.
And since the best compensatory pick available is a third-round pick, the Patriots probably decided that a 2011 first was a significant upgrade from a 2011 third.
It is possible, though, that having two first-round picks in 2011 may allow them to, in essence, trade into the first-round of 2010. Using their newly-acquired first-round pick in 2011, and one of their high picks in 2010, they might convince another team to cough up a high pick in 2010 (it would be infinitely amusing, of course, if that team were the Raiders).
The immediate effect for the Patriots is unclear. On the one hand, the Patriots have traded away one of their best defensive players, and have an extra roster spot to fill, but on the other hand, they've gained $3 million in cap room (to squash one rumor that's been suggested, though, it's unlikely that the Patriots can trade for Julius Peppers: they would need to have $16 million in cap room, when they only have about one-third that amount, even after trading Seymour).
Clearly, though, there are more moves afoot from Foxboro.