No, but the agreements in place more or less guarantee it. Back with the original iPhone, Apple was able to push Cingular into ceding a lot of control over devices. Google was never able to do the same. Partly because they needed to make up marketshare, but also because it went against a lot of the Android ethos (IMO) that pre-dated even the iPhone.

I love open-source, and I'm a huge believer in the possibilities it represents. But I think those beliefs blinded people in the beginning, who assumed that carriers and manufacturers would similarly care. They don't. Hell, they're even slow to respond with security updates. Google is moving in a better direction, but it'll take a while. If we're really lucky, carriers and manufacturers won't notice the gradual changes and raise a fuss.

You'd think that, as long as they get to customize things to their heart's content and install their bloatware at some point, they'd be able to realize the benefits of a better update model. But then again, major UI changes that are clearly visible help differentiate new devices. So there's a nasty vested interest at work there as well.