Nothing stopping MS from extending Skype functionality similar to Google Voice. I think it will happen with Windows Phone 8.1 or 9, since this kind of functionality is likely to be heavily consumerized by then, resulting in a standard commodity service.
Google a'int going to offer any of their services on WP. By doing so, they certify the existence of Windows Phone as a mobile platform among other. While Google is busy building their empire, WP isn't a strategic direction for them.
I generally like this rivalry. We have possibly seen some of the best third party apps on WP that none imagined would happen. Well, creativity has no boundaries and WP is likely feeding some of this creativity in the dev community by being the underdog in this mobile platform race.
Good times, and enjoy the ride