I've run into the problem often where a client using the HTML code for special characters, like ™ for the TM symbol in the product description. Once they go back to edit the description, it turns into a TM symbol, then the page will not validate with XHMTL. Same thing happens with & for ampersands. You can enter & into the description, but the next time you go there its back to &.
Is there any way to force the default text editor to use the HTML code instead of the symbols?