I think Google probably sees it as a possible long term threat, but I think the reason they don't develop for it is because of the tiny market share.
Google develops for iOS after all, and that is much more of a threat than Windows Phone. Google cares more about how many people they can access to target for advertising. That is their bread and butter. That is why they develop for iPhone.
The real reason we aren't seeing much Google development for Windows Phone is mostly that they feel there aren't enough users to justify it yet. I do think there is a certain amount of festering Microsoft hate that is a factor here too. Google and Apple on the other hand didn't always have an adversarial relationship.