Well, had it not been for the unusability of many Reference apps, I'd have mentioned this first. Any decent and feature-rich screen reader should provide a pronunciation editor. All TTS engines have their own pronunciation problems or quirks and VoiceOver's TTS engine (called Nuance Vocalizer) is no exception to the rule. Take, for instance, the word "purchases" which is mispronounced as "per cases" by VoiceOver's US English TTS voice.
So I suggest that a pronunciation facility similar to that of the Mac be added to VoiceOver on iOS. It seems to be much more doable than what Apple have fabulously done with the labeling of icons in iOS 5. Also, given the accessibility of iBooks and the presence of apps such as Read2Go, a pronunciation facility is even more beneficial and urgently needed for VoiceOver.