this single line code will add that functionality to .ahk scripts too!!
RegWrite, REG_SZ, HKCR, AutoHotkeyScript\shellex\DropHandler,,{86C86720-42A0-1069-A2E8-08002B30309D}
Chris, i'd strongly suggest adding it to default installation of AHK. i've used it for ages with .aut files and then with .au3 files ...it has no side-effects.
or maybe i can add it to the TextPad syntax installation script...if u feel for some reason it'd b better.