There’s a rumor today that Sprint, which heralded Palm’s Pre smartphone as an iPhone rival, will get swallowed up by T-Mobile. The deal is only rumored today, and there’s ample reason to be skeptical. But with all the complaints about ATT service to iPhones, this near-swallow of Sprint sparks a closer look.

The smartphone business is run on the backbone of cellular carriers. Make the most wonderful phone you want, but it still relies on the broad shoulders of broadband connections via cell towers and wi-fi. ATT has troubles in NYC and San Francisco serving iPhones. That’s what’s kept the rumors of an extra iPhone carrier for 2010 alive.

Meanwhile, the third largest carrier wants to buy the fourth largest, and T-Mobile will give you a Blackberry Curve 8900 if you just switch over to them. Well, a 1-cent deal at Amazon, but $99.98 less than that iPhone rival Pre sold by Sprint.

People who dislike a carrier often are operating on second-hand testing. I’m in my first month of iPhone use and find it to be no worse than ATT service while I used an ancient Nokia 3120. Far better, when I was traveling the streets of Vegas recently on a visit to my elderly mom (honest!). Google Maps worked great, sort of a pseudo-GPS that didn’t speak aloud. Sprint customers will be speaking loud if T-Mobile succeeds in its quest to own Sprint’s customers. That matters to iPhone users, because the Pre needs a bigger carrier than Sprint to make Apple improve the iPhone.

It’s a Very Good Thing to have a worthy rival to Apple if you own an iPhone. The Pre has the potential, if Palm can survive more losing quarters and someone else gets on board to carry signals from the clever new competitor. I’ve seen the phone in the Sprint stores and admire its design and OS alternatives. You gotta love and cat-and-mouse Pre is playing with Apple over iTunes support. Pre appears in iTunes, then not in an updated version. Then again with a Pre update it appears. Then with the rollout of iTunes 9, no Pre anymore. iTunes 9 is significant because of its better sync capabilities with the iPhone. The 8.2.1. version was all about denying Pre its chances to become iTunes-aware.

The rivalry gives Apple incentive to invest in your smartphone investment. You choose iPhone and get into a two-year marriage with ATT, after all. But this rivalry is about a lot more than the hardware, iTunes, or the WebOS in the Pre. You need a great network to follow you into the suburbs and beyond. I have learned that using the iPhone on the slower EDGE network (more like 2G) is not a joy-joy experience, and there’s lots of places where the joy can disappear.

I’ve had my cell carriers gobbled up by ATT twice and finally succumbed to the assimilation and became part of the Borg. It’s not so bad, at least not so painful that I ever wanted to cancel my plan. Can you choose a carrier as a durable alternative, along with a different smartphone? Only if the carrier is of a certain financial health and a sustainable size. Sprint is scrappy and different, but that doesn’t count in the cell market the way it does if you’re Palm or Apple, developing phones or App Stores. You need a critical mass of customers, flat-out, to survive if all you’re selling is bandwidth.

If the merger goes through, the viable smartphone choices will ride on the shoulders of ATT, Verizon and T-Mobile. (And that Blackberry ain’t bad, if you want to trade a smaller screen for real keys you can press.) I believe that whatever technical hurdles exist to merging their networks, acquiring customers is the mantra for today’s smartphone carriers. None of this is a good reason to switch to a larger carrier. But no matter what your plans are as a smartphone owner, you may not have a carrier choice that will last the long term unless you’re picking a major, healthy player.