Another one. As I was just typing a message on my phone my a, o, and t key wouldn't work. I find this kindof strange for an onscreen keyboard. It's done this before with other keys so I would close the keyboard and reopen, but now that doesn't fix it. It's very sporatic as well happening to about 50% of my messages I type.
Could be your touch screen going bad. If you call tech support they will have you do a factory reset (menu>settings>privacy>factory data reset), so you may want to try that. I am also making the assumption that you are using the stock keyboard, and not one from the market. If this is not so, you may want to go back to stock. One more thought (before the factory reset), try clearing the data on your keyboard (menu>settings>applications>storage use), scroll down to your keyboard (with an HTC phone it should be listed as either touch input or HTC ime), select it, and press clear data.
Hopefully something in here helps, if not, you may be in for a call to tech support, where they should be able to send you a Certified Like New replacement.