Alright, I think I need to emphasize that there is nothing wrong with the N900.
You can manually search for available networks and then try to choose T-Mobile instead of AT&T. Guess what you will see? NOTHING but an error message. WHY? Because as far as I know, there is no national roaming agreement of AT&T with T-Mobile. T-Mobile will block any access to its network if you are using an AT&T sim card.

The phone would roam if there was an agreement of AT&T with T-Mobile on national roaming. Correct me if I'm wrong, but that agreement doesn't exist.

The problem is of course the roaming agreement between two national carriers, and even if it existed, why would you like to pay roaming data fees if not necessary?

Anyhow, the problem (or call it a question) is that N900 does not accept multiple APNs. Only one, sim's default and wireless networks. Or maybe I couldn't find the setting myself. So, is there any possibility to add multiple APNs from the same carrier?

Vote for this "bug" (it's more a missing feature actually):
"Unable to set alternate APN"
https://bugs.maemo.org/show_bug.cgi?id=5791

Somehow the usecase having multiple APN from the same carrier (different payment model, APN overloaded etc.) did not make it into the maemo OS :-)

This is also the reason there is no MMS on the N900. MMS requires a dedicated APN, different from the one that is used for internet connectivity

this thread is a joke right? is imposible to usea a service provider and use the apn from other provider.

While carrier roaming is a small obstacle the real issue is the radio in the N900. It doesn't support AT&T's 3G frequencies. I have the same problem with my E71. It's 3G on AT&T networks but not on T-Mobile, my carrier.

Same issue with multiple APNs. Being able to set only one APN might be a contributing factor but it's not the main issue. It's the radio.

The N900 supports 3G on the 1700 and 2100 frequencies which is the range owned by T-Mobile. AT&T own 850 and 1900. If the radio can't send and receive in those bands roaming agreements are of little use.

If you're thinking about "quadband" phones, remember that while there are "quadband" phones those refer to "world phones" that cover the quadband of gsm voice calls (and, I think, EDGE data). They're not quadband for 3G. According to this article, Sony will be offering a quadband 3G phone http://en.wikipedia.org/wiki/Quad-band. (There's not a lot of data about it yet google).

If you must have an AT&T compatible, open phone the Xperia will be based on Android. Other than that, it's the N900. Personally I prefer the N900 because it's 3G in Europe. I don't travel overseas much now but it's nice to know I can. Might even be 3G in Central and South America which would be a huge bonus for me but not necessary. Just being 3G on not-AT&T is a huge selling point.

Somehow the tech in GSM can set who the preferred and allowed carriers are depending on the area. I've driven across the U.S. a few times and there are areas where a t-mobile SIM is permitted to roam on AT&T, like around the Wisconsin Dells area, as of two years ago (the default was Einstein PCS but I was also able to select AT&T as well). But in other areas I could see AT&T when browsing for networks, but was not able to select it.

A question, related only by the subject line of this thread, not its content:

Is it possible to create additional GPRS connection profiles? Settings > Internet connections > Connections > New only allows for creating new wifi connections. I'd like to create an alternate GPRS connection, to have one with and without a proxy defined.

It's possible to change existing APN in settings by modifying GPRS setting. I do it several times in a day manually. I put the APN address and IP and so on and then if I need operators APN I put the original values there. This should be so that there should be several APN:s or at least some script to help to do that.

Yes, it would save you from having to manually make edits to the one GPRS entry repeatedly.

This was possible on Maemo 4 on my n810. The functionality seems to have been removed in Maemo 5, which is disappointing.