It depends on which "an Android phone" you want to consider. Manufacturers can lock down the phone as much as they like. The N1 however requires one simple co

It depends on which "an Android phone" you want to consider. Manufacturers can lock down the phone as much as they like. The N1 however requires one simple command (which can and probably does void your warranty, depending on how nice HTC/Google feels like being) and then you can flash whatever you like. For other phones, there's usually a nonsupported hack that will let you do the same.