this was answered a while ago, I had to look it up recently when I upgraded to MX 2004:
http://www.phpbuilder.com/board/showthread.php?s=&threadid=10224922
basically:
Open c:\Program Files\Macromedia\Dreamweaver MX\Configuration\DocumentTypes\MMDocumentTypes.xml
Look for winfileextension="php,php3" or macfileextension="php,php3" if you're on a mac and simply add inc to the list so you have:
winfileextension="php,php3,inc"
Save it, and restart Dreamweaver and you should be good to go.
make sure you set your preferences so inc files don't start in code view
Julia