Here’s the explanation I got. Several years ago, Cingular bought out AT&T Wireless (not to be confused with the “new” lowercase at&t). Since that meant that they suddenly had redundant towers (for example, a Cingular tower right across the street from a AT&T Wireless tower), they sold off the towers they didn’t need (apparently they kept the AT&T Wireless towers since they had better equipment, and sold off the Cingular towers).

Apparently, T-Mobile bought those towers, and never bothered to change the towers’ IDs, since it’s purely a cosmetic thing and only shows when a phone or device initially connects. So, when the Sidekick (or any other phone/device) connects to that tower, it says Cingular, and only after it registers with the rest of the network does it change to T-Mobile.

That said, please feel free to take all of this with a grain of salt, since I don’t know just how reliable my sources were.