A few months ago, an FCC filing revealed that the Dell Streak was approved for use on the AWS spectrum. Also known as UMTS Band IV, the Advanced Wireless Services spectrum is owned by T-Mobile and used for the carrier's 3G network (1700/2100MHz), which is why T-Mobile users weren't happy that "UMTS 2100/1900/850MHz" was listed in the tech specs on the Streak's new US product page. Don't be confused by the 2100MHz band; you need to see 1700 for T-Mobile 3G (the carrier uses 1700 up, 2100 down).
Although FCC clearance does not guarantee that the product will actually be released, there are at least a few of the T-Mobile 3G-supporting units in the wild. A member of the Pocketables Forum, for example, has an AWS engineering sample in his possession and says he's seen two others on eBay a few weeks ago (a search of the completed listings doesn't confirm this).
He says the build number and baseband are the same as the AT&T beta devices, so if you're a T-Mobile user and you can find another one of these engineering samples somewhere, then you should be getting fairly stable/mature firmware. You'll also be freed from waiting to see if an official T-Mo version will ever be released.