I'm not one to get overexcited about these kind of rumors. HOWEVER, I would say that there is a certain amount of logic to this one, if you make 2 very big assumptions: (1) Peppers really wants to play for a 3-4 team and is willing to negotiate a reasonable contract, and (2) Carolina has truly given up on Jake Delhomme after the debacle against Arizona. IF those 2 assumptions hold, then trading Cassel for Peppers works for both teams. The Pats free up $14.6M from their cap and can sign Peppers to a long-term deal with a lesser cap hit, addressing the pass rush which is one of their biggest needs. And Carolina saves face by getting rid of Peppers and his $17M cap hit which totally ties their hands, cutting Delhomme and freeing up his salary, and signing Cassel to a more cap-friendly long term deal. Cassel in Carolina, with Gross at OT and Steve Smith at WR, would be a nice place for him to end up.
The Pats would be fairly set at their front 7 for next year. They could either keep AD outside to pair with Peppers (which would be amazing) or move him inside to SILB and use Vrabel/Woods/Crable/etc. And Carolina could focus on keeping the rest of their team intact without the huge cap hit from Peppers, and on finding a decent DE in the draft or FA.
I doubt it will happen, but it's not entirely ludicrous.