Oh, but does it let you select tracks from the screen, or is it just an audio pass-through?
I'm really hoping for a Google Music experience, where you can select tracks and create playlists directly from the Roku.
Unless and until Google can release either their own channel or an API (and they may be blocked from doing so by the music industry) this is probably the best you'll get. Someone could write a screen-scraper to parse the music library webpage, but:
A) That's a lot of work, and it's unlikely you can create a business plan around the idea, so it would have to be a labor of love.
B) Screen scrapers tend to be quite brittle and can break with any change to the website.
C) Google would most likely consider it unauthorized use and demand that Roku block distribution of the channel (as in the case of Youtube).
Mp3tunes (which is already in the channel store, btw) provides an api that developers can use to interface the mp3tunes library to new platforms. Google, Amazon and Apple do not.