Proving that its spine is as thin as its coverage map, AT&T (NYSE: T) will no longer sell unlimited data plans to new smartphone customers starting next week.

We may have seen this coming, given the carrier's notoriously strained wireless network. However, AT&T has the audacity to try to sugarcoat the bad news as a benefit for consumers.

This is the announcement's headline -- and I swear that I'm not making this up:

AT&T Announces New Lower-Priced Wireless Data Plans To Make Mobile Internet More Affordable To More People

Let's assess this from the perspective of Apple (Nasdaq: AAPL) iPhone owners. As of right now, 2G users pay $20 a month to AT&T for unlimited data. Owners of faster 3G and 3GS handsets pay $30 a month for the online usage smorgasbord.

Come June 7 -- the day that most expect will coincide with the debut of Apple's newest iPhone -- the only two plans available will be a $15 DataPlus plan (with a measly 200-megabyte monthly limit) and a $25 DataPro offering (with a better, yet still restrictive, 2-gigabyte cap).

There's a slap for that  
AT&T claims that 98% of its smartphone customers go through less than 2 gigs a month on average, but that metric smells fishy to me. It's lumping email-centric Research In Motion (Nasdaq: RIMM) devices and cheaper smartphones into the mix. I can guarantee you that more than 2% of AT&T's iPhone users are eating through more than two gigabytes of data month.

AT&T's announcement is also unintentionally funny. "Customers can also use unlimited Wi-Fi at home, in the office or elsewhere if available," it reads at one point. Really, AT&T? You're going to let me use my non-AT&T broadband provider's bandwidth without charging me? That's so sweet -- of me.

It's also introducing tethering -- the ability to use your smartphone as a modem to power connectivity for a secondary computing device -- for an additional $20 a month. Uh, what's the point in paying for tethering on a metered plan? If you're going to cap usage, tethering should be absolutely free. Tethering without an unlimited data plan is like taking a yacht out on a puddle, performing a hair transplant on a cadaver, or taking a Lamborghini on a test drive through a school speed zone.

The news gets worse for iPad owners who forked over an extra $130 for a 3G model. Come next week, the $29.99 unlimited 3G data plan will no longer be sold to new buyers. Those who haven't activated the $29.99 plan before Monday will have to settle for the $25 DataPro plan.

AT&T just sold you out, Apple. The iPad is made for heavy data usage. The two most popular non-Apple downloads at launch were video-streaming apps for Netflix (Nasdaq: NFLX) and Disney's (NYSE: DIS) ABC. Who will want to pay a premium for the 3G model that comes with a cab meter?

Opportunity knocks
There can't be losers without winners. AT&T's decision to shutter its unlimited buffet will be a dinner bell for other companies.

Verizon (NYSE: VZ) and smaller rival carriers? We have conflicting reports on the length of AT&T's iPhone exclusivity, but you just found a way to swipe gobs of AT&T's iPhone base, if you can step in with fair pricing and a handset that is compatible on your network .

Sirius XM Radio (Nasdaq: SIRI)? Go ahead and silence the cynics who feel that Pandora and other web-based radio options will kill satellite radio in cars. Consumers will be less likely to plug their smartphones into their in-dash entertainment systems -- instead of subscribing to Sirius or XM -- if they know that every song or sound bite comes at a price.

In short, AT&T is killing the smartphone revolution before it really had a chance to get started, by tripping up its star player.

If you can hear me out there, Apple: Run!

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Longtime Fool contributor Rick Munarriz has an iPhone loaded with free apps, and maybe 4-5 that he has paid for. He does not own shares in any of the companies mentioned here, except for Disney and Netflix. He is also part of the   Rule Breakers newsletter research team, seeking out tomorrow's ultimate growth stocks a day early. The Fool has a disclosure policy.