You may not think that is how it work, but thinking about how something works and knowing are two quite different animals.
From the FCC
"?Roaming? is the term that describes a wireless phone?s ability to make and receive calls outside the home calling area under your service plan. Roaming occurs when a subscriber of one wireless service provider uses the facilities of a second provider. While the subscriber usually has no pre-existing agreement with the second provider to handle calls, the subscriber?s provider may have a ?roaming agreement? with the second provider. Under that agreement, the second provider agrees to handle calls placed by subscribers of the first provider and vice versa. When your phone is roaming, an indicator light on your phone may display the word ?roam.? On occasion, your handset will not display a roaming indicator, even though it is in a roaming area. Also, some handset software needs to be updated monthly. Often this can be done by simply pressing a few buttons on the handset. Keeping that software updated can increase reliability and reduce incorrect roaming charges."
When you are roaming you are using someone elses access points. The "coverage" maps are loosely based to display where coverage occurs for the access points belonging or affiliated directly with the service provider.
Very rarely have I been dropped from someone elses network, whether with my Cingular, Verizon, or T-Mobile phones. (PDA, Work, Home). Most cell-phone towers interoperate with each-other quite efficiently, though I have seen underhanded tactics like getting low-prio. on calls made from Service provider X while in the vicinity of Z. For example, X phone gets "All circuits are busy 75% of the time" while Z phone goes straight in. This is still a rare occurence.
The best display of this is when I took my T-Mobile to Guam. Guam does not have T-Mobile or very many other networks. I called Guam Cell to ask if it would work before checking and they will tell you that you are roaming on their network and using their services, and will be charged big fees and to us their service which charges long distance to call back to Mainland.
I turned on my phone, got the ROAMING tag, hit the web and saw that I was not going to be charged for ROAMING. I used my phone for more than 900 night and weekend (free) minutes and never saw an additional charge. I constantly called Mainland US and while in the US I always call Guam. Never once have I seen a fee, whether I was in Florida on T-Mobile network, South Dakota on (AT&T or Sprint?) network, or Guam on GuamCell networks.
I have not yet lost signal except in places where my co-workers all lose signal with their carriers as well. And when my T-Mobile is out of range, so are my Cingular and Verizon. Go figure.
Make of this what you want. FCC.gov has a nice blurb on it..