in DW CS3 you need to edit the MMDocumentTypes.xml in C:\Program Files\Adobe\Adobe Dreamweaver CS3\configuration\DocumentTypes folder on windows.
On line 75 you'll find:
<documenttype id="PHP_MySQL" servermodel="PHP MySQL" internaltype="Dynamic" winfileextension="php,php3,php4,php5" macfileextension="php,php3,php4,php5" file="Default.php" writebyteordermark="false">
just add ",inc" after the php5 in either winfileextension or macfileextension properties (or both doesn't matter as long as you change the one for your OS) so it looks like:
<documenttype id="PHP_MySQL" servermodel="PHP MySQL" internaltype="Dynamic" winfileextension="php,php3,php4,php5,inc" macfileextension="php,php3,php4,php5,inc" file="Default.php" writebyteordermark="false">
and restart DW
You can do this with any file extension for any coloring format that DW has too.
To my knowledge this is the only way to edit how file types are handled, i've looked through all the menus and couldn't find anything else.