As someone who recently got a HTC Hero, I had to jump through several hoops to get root access on the phone to install custom firmware. Now, Android is open-source and fairly easy to build and hack on an emulator. It seems to be against the spirit of open-source to lock down a phone so you can't hack the phone itself.
Now, often, there are understandable (though not always justifiable) reasons for locking a device down. For example, it might have proprietary software on it or you might want to retain control of the platform. However, Android by its open-source nature makes such concerns moot. Everyone and their dog has access to the userland code, and HTC is forced by the GPL to release kernel sources for each of their devices.
So, I fail to see any motivation for alienating the hackers, when there is no possible benefit (in my mind) to be had from doing this. Any idea why a company would want to do this? Is it just short-sightedness or am I missing possible commercial implications of this?