I have a two month old MB, and have also noticed the sheath around the cord (where it attaches to the magsafe) is longer and slightly different than the one in the pics here. Hopefully, I won't run into any problems. I already make a point of never leaving it plugged in when I'm asleep or out.
I can't help but wonder about two points in question here:
1. One of the selling points of the MB, is that the magsafe will pull out of the computer if enough stress is placed upon the cord. So in theory, the cord should be able to handle a few tough tugs now and again. If this frayed cord/adapter is actually caused by stress on the cord/adapter, then the whole magsafe spiel is a crock of shiitt.
2. OP, why would you place your MB (when using it) on a neoprene sleeve? In my opinion, this is very careless. I mean, how can not realize that the MB is going to become unnecessarily hot like this during use? If using it on your lap, why not place it on a large book, or tray?