Why?
I have inc and phtml files executing on one site (and php obviously). The other also runs wphp, but not phtml.
Add this to your .htaccess file in the main directory...
AddType application/x-httpd-php .inc .phtml
Just add on whatever extenstions you want, making sure there is space between each.
Oh yeah, but if your running this on a win32 system it probably won't work, at least my ME server (offline) doesn't like them, the Unix ones do though.
Of course if your running a live server on a Windowz system something not quite right :-)