He said it wasn't his service provider...

Any way, I'm having the same problem as well. Ubuntu 10.10 (and previous) constantly disconnects and it's not the provider, it's the phone or Network Manager. After a disconnect, I can access the net from the phone, but Network Manager won't connect until I disconnect and reconnect the phone.

Only way to rule out if it is a Network Manager bug or a Maemo bug would be to use Windows or some other non-Network-Manager-OS, but I don't have any of that. However, if this is the same problem as hteink.min's then it is the phone dropping the tethering since a USB stick works.

As for the auto-disconnect comment, definitely not, I haven't installed anything like that nor would I set it up to do that if there is an option for it. That would be a pretty silly option if there was...

Even if the service provider changed hteink.min's IP, that shouldn't break the connection between PC and phone.

This has been a bug since I got the phone, and sometimes I can stay online for quite a while. Once it even stayed on all night long, but usually I will get a few hours out of it before it drops, sometimes only a few minutes, and sometimes it won't connect at all, and I will have to unplug and replug it in again until it starts working again. I'm using TMobile, but it doesn't sound like this problem is the provider.