Jason Snell has an excellent article called “Time for Apple to open up the iPhone” where he makes a good case to for Apple to allow unapproved apps on the iPhone. He argues quite well that Apple’s reputation is being harmed without any good benefits.
He thinks Apple should do what Andriod does, and have a deep hidden setting to allow unsigned apps, with a giant warning. Most customers would never know, but they ones who did would be aware of the (very small) risks.
Read the whole thing, its good.