Interesting and definitely a good idea. Basically this will cut down on actual Android updates and reserve them for core functions rather than things that could be bundled into Apps. This gives manufacturers more time to distribute their OS upgrades (because they should be a little less frequent), which gives developers less versions to develop for.
Bravo Google.
Isn't Apple doing a similar thing on the iPad with the iBookstore app? If I remember correctly, it's not installed on the iPad out of the box. It's an app that you download through the App Store. I read that it's set up this way so that they (Apple) can push out updates to the program at any time, without having to release an entirely new OS version. This seems like how they should also do the "core" apps like maps, mail, Safari, etc. Would be much easier to update/upgrade the apps that way, when all they would have to do is push the updated app to the App Store instead of hammering their servers each time they upgrade the OS for minor tweaks to core programs.