Google Code Jam Registration Opens Today
the thing is, the topcoder infrastructure supports certain languages and it costs them $$$ to implement additional ones. When there is a payoff to adding Ruby, I'm certain they will add it.
Using Python is problematic anyway as they state that due to slow runtime of python, some problems won't be solvable. They will time out. I expect this would apply to Ruby also. And I would hate to invest the time to solve something in Python/Ruby only to find out that it won't run, then have to port it.
On the other hand, if the productivity claims of Python/Ruby are true, then a more fair contest might weight time to complete the problem vs. runtime. E.g. if you implement a correct solution quickly, then you get more runtime to test. Something like that.