the regular doctype is fine for php forms.
I am not able to reproduce the problem. I just tested using IE 8 and IE 9 and the forms are submitting fine.
one thing that may be throwing people off is the reset button, are you sure they are clicking the submit button and not clicking the reset button?
I made the mistake of hitting reset button, because i am used to seeing it on the other side. I caught myself though and used the submit button.