There's a suggestion on uservoice for this, you can vote on it if you're so inclined.

Last we heard from Waze, they were not currently planning to develop for WP7. (I think) The reasoning is that it is not really backwards compatible with WM... it's really an entirely different OS. That would make it quite time consuming for them to develop for it. They've already got their hands pretty full.

I just don't think that WP7 is going to gain a particularly huge following, honestly.