Their unlimited data plan (for surfing the web, media, etc.) is very expensive."
It's not just the Lumia, and it's not just AT&T.
It's EVERY carrier, and EVERY smart phone. Don't take offense at all when I say that you must not have been shopping for a smart phone in the last few years.
What truly chaps my hide about the carriers (AT&T, Verizon, Sprint, et al) is that ANY such device must have its own data plan. IOW, as a Verizon customer, I have a smart phone (Motorola Droid; Android OS) with a data plan. Fine. Now, if I go and buy a tablet with wi-fi and 3G or 4G capability, yet ANOTHER data plan if I choose to avail myself of the 3G or 4G capability.
It's a ripoff, and it's high time someone offers an all-encompassing data plan tied to the USER, not the device(s).
No, T-Mobile unlimited is cheaper than AT&T. Which is why I can't consider the Lumia 900.