iPhone coming to T-Mobile in Q3?

Whenever the rumor mill gets to chatting about the iPhone heading to another carrier beyond AT&T, it’s almost always focused around Verizon. On the off chance it’s about someone else, it’s generally Sprint. Oddly, the one most oft forgotten is the one that, from a technical standpoint, would be the easiest transition: T-Mobile. AT&T rocks SIM cards — so does T-Mobile. The only thing keeping a T-Mobile iPhone from happening is the lack of support for T-Mobile’s 3G bands — and AT&T’s nasty little exclusivity agreement, of course.

That exclusivity agreement is often said to be coming up on the end of its lifespan — and according to the latest whispers, T-Mobile is waiting with open arms.

The word comes through Cult Of Mac from a “highly placed” (aren’t they all?) source at T-Mobile. According to their tipster, talks are “in the advanced stage”, with a T-Mobile iPhone “about 80% likely” to launch in Q3.

Now, I’m not one to chalk up rumors as nonsense on a whim, but I’m also not one who likes to get everyone’s hopes up for no reason. We’ve heard this one before. Lots, and lots of times, for each and every carrier. You can bet your arse that the iPhone will be spreading out to as many carriers as possible the second AT&T’s exclusivity agreement is up, but for all we know that agreement isn’t up until 2012. There’s always talk that such-and-such loophole is letting Apple escape the agreement early, or that a new agreement has been reached — but so far, they’ve all been the pipe dreams of would-be iPhone owners waiting on the edge of their seat but refusing to jump ship from their carrier.

(Tangent: Also, where the heck does someone pull a number like “80% likely” from? How can something like this even be measured by probability? Silly.)

Think its going to happen? Where would you pin the odds? If I had to guess, I’d wager it’s somewhere between -312% and 104%. Let us know in the comments below.