It's going to be another year at least (it'll probably take that long for the merger to go through) before 3G is switched to AT&T frequencies, might well be longer because they won't do it straight away after the merger has gone through either, I would expect them to give T-Mobile customers time to actually upgrade their handsets to ones that will work on AT&T frequencies. So you've got a while before you need to start worrying about it. When it does happen, what might be an option is getting a data-plan with a MiFi-type device so you can still have a 3G (or maybe 4G) connection to your N900. Or possibly a new phone that supports WiFi tethering and keeping the N900 just as an internet tablet.