qwert: Good point - it seems, on first glance, that when the data is bound (e.g., if you are doing simple binding), you can format on binding...
...however, I am not familiar with using a Format string within a textbox - something tells me that it can be done; but the only examples I can find (not at home now, so no library in front of me) is to format on binding.
Good question though. I suspect you could use JavaScript to accomplish this. It'll be interesting to see how this works out.