Not perfect, but pretty good.
I'll admit you got something going here on the right track. Your use of 3D and 2D was done pretty well, and the cat didn't look out of place much, if at all.
You do have some things to work on though. I think the cat's anatomy was a bit funky when you had it running or walking. Also, there wasn't much emotion expressed...the cat constantly looked stunned or confused with it's mouth always open like that. I think it would have been sooooo much better if you moved it's mouth to mimic it's emotions better, or at least keep it's gapping hole shut XD But you did express it's sadness in the end!
The music was fitting, but a few of the sound effects seemed kinda tacted on or out of sync, or even not really fitting. But this was one of the lesser issues I had with it.
It kinda made me wanna tear up at the end. A very well done flash overall! Keep up the good work! I can see this making front page easily!