Font website iOS Fonts has put together a growing list of fonts available in iOS 4.2 for iPhone and iPad.
According to the list, there are 40 font families available for the iPhone and 57 font families available for the iPad. There is no mention of the iPod touch, but the available font families in the iPhone probably match the iPod touch.
The list is rather interesting in that there is a difference of font families between the iPhone and iPad on the same iOS. In any case, the list of fonts is a great resource for iOS developers and designers alike.
At this time, these fonts do not seem to be available to general users of iOS and are instead used for development of apps, for example.