AT&T-Windows Phone 7 ads for HTC's Mondrian leaked out earlier this week. Microsoft is expected to use its annual event in New York City on October 11 to showcase Windows Phone 7, among other technologies. The US will see the actual devices arrive on November 8.
It's not surprising that AT&T is a launch partner, given that Windows Phone 7 is limited to the GSM standard until 2011. That said, exclusivity to AT&T means that T-Mobile won't have Windows Phone 7 at launch, maybe a little more of an eyebrow raiser.
It's worrying that Microsoft's thinks it's a good idea to limit its carrier options to just one in the US. Sure, Apple did it and the iPhone was still a success, but that was another time. Now the market is much more saturated, and one of the reasons Google's Android is growing so quickly is because it's available on multiple carriers.