It would depend on what is in the firmware but I would have no problem with it... It would be similar to a commercial desktop OS. If paying guaranteed new features and support I would have no problem! Mac and Windows (among others) currently employ this so why not extend to the mobile platform.

Apple are already doing this, not with the iphone but with the ipod touch, the last update was a $10 charge to get it for the touch and I am sure they will do the same again with 4.0...

A fiver (pounds) or less than $10 us is nothing for a quality community backed functional OS.