Why people don't buy TabletPCs
Engadget has a short piece by a Jupiter Research analyst that explains why TabletPC hasn't done as well as many expected. Conclusion: people don't care about the price, but they don't feel (or don't know) they need the extra features.
Note that there's no mention of speech at all in the responses Jupiter got to their survey. People think about handwriting recognition, note taking, and even the hi-res screen before they think about speech.