There is a free speech recognition app for the iPhone and iPod called Dragon Dictate, but it has a lot of errors. Someone with really good speech might be able to figure out how to make it work, I suppose.
Dragon Dictates makes a full-powered speech recognition software program that needs to be trained by the owner of the program. Prices have gone down a lot----I saw it on sale for $39.99 at Costco, I think. The technology is still a very long way from being able to have a small portable device recognize anyone's speech on no notice. That's why we still need to hire providers of CART (Communication Access Realtime Translation) for conferences and teleconference and why television networks have to pay a lot of money to realtime captioners to caption live programs. These services are expensive since it takes years for the providers to get really good, and relatively few people are skilled and experienced enough to do really good work.
There are a few mobile phones that will work with Web CapTel, a specialized kind of VCO relay service that is normally used by people with hearing loss who can speak for themselves. Like any relay service, it's not intended to be used for in-person communication, though. The service is a bit slow, tends to lack a great deal of punctuation, and can fail to produce even approximate text for unfamiliar names and words. It's helpful as a back-up but definitely has limitations at this time.