I would also like to know how to change this, I bought my HTC 7 Pro from "Media Markt" in Germany, and I have the same problem.
But I'm not sure if it's a good idea to do so, because even if I did change the Keyboard Layout from the software side, the hardware will still be QWERTZ, so what I press will not be what appears on the screen :S
BTW the differences between US QWERTY and German QWERTZ isn't just the Y and Z keys, other differences include:
( and ) are at 8 and 9 istead of 9 and 0
& is at 6 not 7
* is at 7 not 8
the semi-colon (drove me crazy when coding at the university there)
and a lot of other punctuation were moved away to other buttons to clear for the "umlauts" and stuff :S