I know what you see in the screenshot, but don't jump to conclusions.
Although T-Mobile's presence is undeniable on a Dell.com page that mentions the Streak, the two tabs ("T-Mobile plans" and "T-Mobile Customer Agreement") are probably just on the company's rate plans page because of the Dell Venue Pro, a Windows Phone 7 handset destined for T-Mobile.
The confusion, if there is any, comes from the fact that the Venue Pro isn't actually listed on the page. The Aero and Streak are there and correctly identified as being available specifically with AT&T, but I suppose it's easy for your eyes to miss that part when they hone in on all the T-Mobile info.
Click for larger view.
There hasn't really ever been any chatter about the Streak being sold with a T-Mobile contract, anyway, so I think it's best to pay no mind to this and just keep moving.[Dell Mobile Device Rate Plans] Thanks, Chad!