A Sprint iPhone still seems like a pipe dream, even though there are plenty of reasons for Apple to release a device with Sprint.

In two days at this week, there was one thing Sprint employees absolutely, positively did not want to talk about: any rumors of a Sprint iPhone.

Sprint's product chief Farid Adib didn't smile about the Sprint iPhone chatter, the way various Sprint execs did when journalists brought up the idea of . In fact, everyone in Overland Park looked a bit weary and a bit irritated about the topic.

But analysts keep wishing, hoping, and asking about the product, because at a basic consumer level, the lack of an iPhone on Sprint doesn't make sense. Sprint uses the same 3G network technology as Verizon, on the same frequency bands, although Sprint made it clear its phones are tuned and optimized differently. Still, Apple wouldn't need to build new hardware for Sprint, unlike with T-Mobile USA, which uses a frequency band not available on any current iPhone.

In other words, this looks like a business decision rather than a technology issue. Most people, including many Wall Street analysts, are saying: why the heck not? It wouldn't require new hardware, and it would open the iPhone up to millions more people. Here are some theories as to why it hasn't happened yet.

1. Verizon has an exclusive, even though Apple says it doesn't. You'd think Apple would be sick of long exclusives, after being trapped in AT&T's embrace for four years. But Verizon Wireless has a lot of bargaining power. A six-month exclusive would have pushed a Sprint iPhone 4 into the summer, where it would risk running up against any iPhone coming out this fall. Apple COO Tim Cook contradicted this theory at the Verizon iPhone launch when he said the two companies had a "multi-year, non-exclusive deal." But who knows how that language could be parsed?

2. Apple disdains Sprint. Apple execs tend to have personal feelings about U.S. wireless carriers that they don't have about international carriers. For Apple, brand is everything, and the company doesn't want to be associated with brands it sees as "losers." it wants to be a winner, associated with winners. This has worked well for Apple in the past. Apple execs could simply see Sprint and T-Mobile as hopeless loser carriers headed towards extinction, and not worth the company's time.

3. Apple is waiting until it needs a boost. Every time the iPhone comes to a new carrier, Apple sells several million more iPhones. Apple is doing very, very well now; its quarterly net income . Perhaps Apple is leaving the Sprint arrow in its quiver to goose its revenue in a slower quarter.

4. It would imperil Sprint's role as the "unlimited carrier." Sprint is the only remaining major carrier with a truly unlimited smartphone data plan, as CEO Dan Hesse underscored in Overland Park. iPhone users are notorious data hogs. Perhaps an iPhone on Sprint would have such a painful effect on Sprint's network, the company has chosen to keep its unlimited philosophy intact rather than introduce new plans to throttle the iPhone-carrying hordes. (I don't really agree with this theory myself; last year, Validas did a study showing Verizon Android phone users sucked down more data on average than AT&T iPhone users.)

Why do you think there's been no Sprint iPhone so far? Offer up your theories in the comments below.