Alternative implementation of isType()
I could not find anything wrong with your code,
so per Meta I am going to tell you why it is awesome ;)


Checking .toString() is icky, toString() could easily be replaced/enhanced
Your solution works across frames
I was not able to break it.


On a final note, it seems to me that needing to verify the type of an object ( besides assertions ) means you are doing something odd and probably the code should be re-thought.

Still, I am favouriting this question.

UPDATE : Turns out that according to jsperf, your solution is much slower :\