Let me lay down the basics first. The Google Phone has been labelled a certainty and confirmed by two reliable rumor sites. Google itself has denied it, but that doesn't necessarily mean anything. Or maybe it does. Such is the vague nature of the tech industry. Anyway, there are also some rumors that this purported Gphone will be an entirely data-driven device. This would reduce its dependency on carriers to a minimal level; you'd just need a data plan.
Today, TmoNews is claiming that a "trusty" source has informed them that T-Mo will be supporting the G-phone, which is currently codenamed Passion. Possibly for the HTC Passion, which we now know exists, thanks to a leaked Android 2.1 ROM.
TmoNews is usually a reliable source for information related directly to their company. They're also far from an objective site, and may be giving a ridiculous, obviously fake rumor a lot more credit than it deserves. It's sort of hard to tell. If Google is building a phone, which seems likely, HTC would be a reasonable choice for a manufacturer. The two companies worked together before on the G1.
I suppose T-Mobile is as likely a first carrier as any for the Gphone. However, there is no chance at all that Google will have exclusivity with any one carrier. That would lose them a lot of credibility with their most dedicated fan base, and go against one of the core values of their whole company. Besides, if the Gphone is data-driven, limiting that data plan to one carrier alone makes absolutely no sense.