If it's Verizon that's behind this to "protect the network" it's starting to sound like we're heading for another "Carterfone Decision":
https://secure.wikimedia.org/wikipedia/en/wiki/Modem#The_Car...
The Nokia N900 doesn't make life difficult for users. To root it you just download and run 'rootsh'. Done. The network hasn't exploded simply because it's easy to root and update ROMs.
What kind of harm? I don't see any harm you could do with this phone that you can't do with another / an antenna. And if it's P.R. harm they're afraid of, people who replace their OS / jailbreak are usually proud to claim they did so to anyone in the vicinity, so any bad experience on their part isn't spread to others from what I can see.
Remember the failure of Vista's security dialogs? And how everyone just kept hitting "yes"? Same story. The average user wants it to work, not to not work. If A and B are required for it to work, they'll add A and B without a second thought.
Just saying - much as I pity the average user for their helplessness, but their is a limit to the level they can be accommodated.
Computers do not require a user's-license in the least, and it shows in how much BS support lines have to put up with.
If you install a hacked OS, you don't have that security anymore. Unless you can trust the provider of the OS.
Conversely, a lot of people have modified their Android phones, yet this doesn't seem to happen to them.
Because of the lack of correlation between any of these things (hacked phones, phones misbehaving and network disruptions), I have a very hard time believing this nonexistent problem is really the motivation for these policies.