If there was a market for it, people would be making them, if you disagree with that they what are you doing on here, your fortune awaits...
Also worth seeing kids who've never used a physical keyboard to any great extent. I've seen them doing what must be close to 60 words a minute on a touch screen (tablet rather than phone but still). If you're used to it a touch screen keyboard isn't a massive limiting factor.
EDIT: This is interesting: http://psychology.wichita.edu/surl/usabilitynews/122/ipadtyp...
A study showing netbooks to have a typical typing speed of around 60WPM vs 40WPM for iPad, but interestingly it mentions that none of the participants had used an iPad.
I'm guessing that someone with significant experience on a particular OS / soft keyboard with correction could close that gap pretty sharpish.
EDIT2: http://thinkertry.com/2012/10/30/typing-speed-test-iphone-vs...
Someone testing iPhone, iPad and keyboard two years running:
2011: iPhone: 57 wpm, iPad: 60 wpm, Keyboard: 71 wpm 2012: iPhone: 51 wpm, iPad: 75 wpm, Keyboard: 82 wpm
It seems that practice does indeed close the gap. Given that the total install base for touchscreen devices will exceed that for PCs in the next 12 months (probably next 6), that greater level of familiarity will shortly become standard.