My review of data coverage in Oscoda, MI:

Cingular: Dead to the world. My phone is displaying "Emergency Only". No amount of roaming agreements makes it work here. Perhaps they should have a lunch with Centennial wireless and be like "yo, what's up! We need to talk!". I can't even get voice coverage here.

T-Mobile: Roaming on Centennial Wireless with full signal. (T-Mobile does not charge to roam with data services) The GPRS is not EDGE, but is the expected 56k(ish) that comes with straight up GPRS. yay!
I should note like all GSM services, the roaming is absolutely, positively transparent to me, the end user. The only difference I notice is that my phone doesn't say T-Mobile on the screen. SMS, MMS, Data, etc, work great.

Sprint: Dead. My phone sits happily roaming (though it doesn't matter, since my Sprint plan makes roaming irrelevant) on either Alltel or Verizon (not sure). My data service does not work, as CDMA really blows ass like that (CDMA's roaming facilities don't support roaming of special features like voicemail indication, MMS, Web access, etc), though much to my happiness, SMS is working great, bidirectionally even, which makes me really wonder why voicemail notification fails to work (though perhaps this is because in GSM Voicemail notification is a special form of SMS, I have no idea how it works in CDMA)
Of note, the roaming on my sprint phone is digital, as my phone does not posess an analog stage. It is behaving quite nicely as a phone, save for the irritation of no voicemail indicator.