Nuance just released a dictation tool (Dragon Dictation v1.0.1) for the iPhone. http://www.mobilecrunch.com/2009/12/16/nuance-updates-dragon-dictation-app-to-let-you-keep-your-contacts-secret/
Generic transcription of speech to text
Google Mobile App for the iPhone and Nuance are the only tools with favorable ratings (ie mostly 4s and 5s). You would expect Microsoft/Tellme to have an offering too, right? Strangely, as of Dec. 2009, they still don't have an app for the iPhone.
VoiceOnTheGo http://www.voiceonthego.com and Dial2Do (http://dial2do.com) are smaller players with similar offerings.
Keeping in touch with your friends and family (contacts ie)
Infinear http://www.infinear.com focuses on proper nouns (your father's name, mother's name, children's, friends', restaurants...) are EVEN more important to an average user than generic words. If I can put you in direct contact with a person whose name you can say, then there is no need for transcription. We store the entire audio on our servers. Every client (including your phone) can play back these messages for offline access. Hooking all this up via Yahoo Mail or GMail opens up the largest set of collaborative users on this planet-267 million yahoo users and 100 million GMail users ie.