Speculation over the iPhone coming to Verizon Wireless will continue until it actually happens. Loyal Verizon customers want iPhones, and AT&T continues to take it on the chin. The promise of a huge untapped market is enough to keep analysts and investors buzzing indefinitely.

Apple’s exclusivity deal with AT&T is the most obvious barrier, and it’s what most observers continue to scrutinize. But even discounting that obligation, there are still a number of obstacles to overcome before an iPhone shows up on Verizon’s network. Here are a few:

Simultaneous Voice and Data

Think about the number of iPhone commercials you’ve seen that demonstrate using apps while a call is active. Now imagine those commercials vanishing because the iPhone is on Verizon. The ability to—wait for it—multitask while on the phone has been a critical selling point of the iPhone experience since the iPhone 3G launch almost two years ago. CDMA’s answer for this long-standing weakness—SVDO—was only announced last August. Verizon seems to be betting on LTE, but we’re still waiting.

Apple is not the kind of company that takes huge steps backward just to win a few more customers. Don’t hold your breath for a Verizon iPhone until this problem is solved. I remain baffled as to why AT&T has not been more vocal about it.

Features and Services

Technical limitations notwithstanding, Verizon has a long history of forcing partners to remove or alter smartphone features, and add its own. Marco Arment talked about this just yesterday. The story appears to be improved in the Droid era, but it’s still a likely point of contention in any Apple-Verizon negotiations: iPhone either works the same on Verizon as it does everywhere else, or not at all.

Surely AT&T or Verizon wouldn’t mind boasting that the product can do more on its network, but it’s not AT&T’s or Verizon’s product. iPhone has spent three years defining itself in the US. To suddenly confuse that definition would be a problem.

Retail

Right now, every iPhone in the world uses GSM technology, regardless of the carrier. Adding Verizon would mean building, testing, and forecasting new hardware for a CDMA model. It would also have a significant impact on the well-oiled Apple Retail machine.

Every time a new iPhone launches, there are lines out the door. Add a CDMA phone to the mix, and you either have two lines, or complicate the process by waiting for each customer to decide between carriers. If the customer changes his mind mid-purchase, the Apple Retail rep has to go get the other model—adding time to the transaction and running tempers higher for everyone in line. In-store returns will inflate with gifted iPhones that were bought for the wrong carrier. The retail implications of a second wireless chipset are in many ways negative.

That may sound like a minor issue, but it’s the kind of thinking that makes Apple a different company. The Apple Store arguably boasts the best retail experience in any industry. Anything that compromises that experience is not likely to go over well.

Branding

Did you know Verizon invented the Blackberry Storm? Have you noticed it’s always listed before Google and [insert OEM here] in Droid ads? Like most carriers and telcos, Verizon believes it brings, well, everything to the table, and subsequently demands all the credit. Apple, meanwhile, is the only Intel partner on the planet that does not have an Intel Inside logo anywhere on or near its products. If any talks between these two have taken place, you don’t need a vivid imagination to guess how they’ve gone.

Apple: Sorry we haven’t talked in a while. Where did we leave off?

Verizon: We were discussing where our logo would fit on the back of your phone.

Apple: [click]

This will be a deal breaker if it isn’t already. Guess which company will have to give in.

As with any potential partnership, there are a lot of things to be worked out here if and when the time comes.