I thought that Nick's shows were basically aimed at little kids to begin with.
Anyway, this seriously surprised me. I thought that TMNT would get another series, but I didn't think this soon. I actually see more good things about this situation than bad. For starters, this deal probably helps out 4Kids quite a bit. Since they wouldn't have been able to make a new season/series, they wouldn't be able to make much from the series and the toy sales have been kind of poor lately. Plus, they may still be unable to make a new series when the movie comes out. Considering that their series was never a big hit, the money they got from this deal seems like it would be helpful.
I think that Nick will do a good job with the series. Sure, outside of Avatar, they haven't had that many successful action shows. Nick is better at handling comedy shows. Still, they could make it work on either Nick or Nicktoons. Based on how they handled Avatar, which wasn't perfect but still really awesome, I'm sure that they'll be able to do a good job with this series.
Since Nick will get rerun rights to the 2K3 series, there's also a good chance that they'll re-release that series as well.
The only bad news I do see is with the comics. I've never read the comics myself, but if that is stopping, then I do feel bad for the people who have and enjoyed the comics.
Still, this is shocking, yet awesome, news. I would be up to see another TMNT series and I'm sure that it will be fun.