As I understand it, the case with the talking cat (and other things that showed up in that episode) is (SPOILER)Haruhi made it happen because she wanted it to. She wanted the cat to talk, so it did, just like on one of the parts, Mikuru attempts to "use" the Mikuru beam and it actually happens, again because Haruhi wanted it to.

Also, apparently they've confirmed that a second season is in production, it's supposed to be out in Fall 2007.