America's number three and four carriers offer unlimited data plans, and that can't be bad, especially if that means more Carly in leather TV commercials.

Competition is a good thing, and if AT&T had taken over T-Mobile then it’s unlikely that we’d see the kind of bottom-dollar pricing on unlimited iPhone data plans available today. More important still, there would be no TV commercials featuring Carly in that super hot, leather motorcycle outfit – but I digress.

T-Mobile is the last major carrier that doesn’t sell the iPhone and are bleeding subscribers to the bigger providers as a result. Yes, little fuchsia claims 1 million iPhones are on their network, but all of those came from somewhere else, namely AT&T.

In an effort to attract new subscribers, T-Mobile has announced plans to offer unlimited data plans from September 5. That sounds great, but the bottom line is how do they compare with comparable AT&T, Verizon and Sprint plans? Here are the basics:

Subsidized phone, unlimited everything:
• T-Mobile — $89.99
• Sprint — $89.99*
• Verizon (data cap with throttling) — $110
• AT&T (data cap with throttling) — $110

Notable cheap plans
• T-Mobile (Value, bring your own phone) — $69.99
• Sprint (Everything data, 450 minutes) — $79.99

* Bloomberg estimates a comparable Sprint package would cost $109.99

“The bottom line is [T-Mobile has] capacity available to attract quite an influx of subscribers without pulling down the network,” Steven Crowley, an independent network engineering consultant, to the AP.

T-Mobile are currently saying that they will complete a 1900MHz HSPA+ rollout by the end of 2012, which means the iPhone 4 and iPhone 4S are or will soon be compatible across their network. That said, many T-Mobile coverage areas have already been upgraded — call your local fuchsia retail outlet.

Devilish Unlimited Data Plan Details

Another thing to consider when shopping for an unlimited iPhone data plan is the fact that T-Mobile (GMS-compatible) isn’t currently charging for FaceTime (how to activate) and likely won’t. For what it’s worth, Sprint (CDMA) says they won’t charge more for FaceTime service.

So, if Apple’s video chat service is important to you, here’s another reason to avoid AT&T.

T-Mobile isn’t yet offering true unlimited data plans (they’re coming September 5) and the full details aren’t yet clear. Nevertheless, paying less for more on a network with decent capacity, especially if you can bring your own phone, sounds like a very good deal.

What’s your take? Would you consider switching to an unlimited data plan on another network to save money?

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