Nuance did an interesting study of mobile phone users that has a few intriguing conclusions:

• Americans typically use 450 min / month; UK uses 200.
• Only about 20% of users think voice-dialing is "not very helpful"
• Half of mobile phone users talk while driving.
• Only about half of MP3 player users thought speech-activated play would be useful.
• A similar number (about half) thought some kind of dictation ability on cell phones would be useful.  In the UK (where 90% of people say they SMS regularly, 70% of users think it would be useful).

The study was conducted about a year ago, so some of this may have changed slightly, but generally I'm surprised at the high usage of voice dialing.  Given the complexity of setup on most phones, I would have expected far fewer people to bother. 

Speech fanatics often think dictation on the phone is the Holy Grail -- just think how much more productive you'd be if you could write emails in the car! -- but personally I question how much normal people would actually use the feature.  Still, I'm surprised that the amount of interest is about the same as the feature I think really would be useful: picking songs by name.