Strike 3 has to be caught by the catcher regardless if it was tipped or not. If it hits the ground the runner can run to first and the catcher will have to throw him out just as if he got a hit.
I hate referring anyone to wiki but it is explained well on this topic.
http://en.wikipedia.org/wiki/Uncaught_third_strike
How about if first base is occupied? Does the catcher still have to catch a strike for it to be an out.