Two clubs going for a win/win rather then a "gotcha."
Miami is rebuilding, their offense sucks and their D is older then the Pats' LB corps. The Pats want Welker, they can just take him using the poison pill and create bad blood, but that's not their way, or hasn't been in past trades.
As speculated above:
One of the Pats' young lineman, such as Wright or Santonio Thomas or Britt or O'Callaghan.
Maybe the Pats want that #10 and are working a deal that sends Samuel & #24 & 2008's #2 to Miami for Welker & #10 (this is an example, there is no attempt to be exact).
Maybe Miami doesn't think it can get one of the top tackles this year, or wants a second pick in the first to increase the odds.
There are a lot of scenerios where both clubs get something they want/need without resorting to a poison pill or trying to cut the other guy's throat. Division rivals or not, it sounds like BB/SP are playing for a win/win to get what they want without resorting to the kind of dirt that made last season so entertaining.