Maybe I'm wrong, but I see an issue with simply using onfocus. If the user types a value in the textbox, clicks somewhere else, then clicks back in the textbox, whatever value he or she had typed in the textbox would be erased. You would need some sort of check to make sure it was only the first focus.
How about:
<script>
var isFocus = 1;
function eraseText () {
if(isFocus == 1) {
window.document.blahform.blahtext.value = "";
var isFocus = 0;
}
}
</script>
<form name="blahform" id="blahform" action="#">
<input type="text" name="blahtext" id="blahtext" value="Type Here!" onfocus="eraseText();" />
</form>
I apologize for my malformed JavaScript and the fact that I may have made a few mistakes in the code, but that's the general idea of what you should have.