Online petitions are a waste of time. There is no company who has ever been convinced to do something because of all the "signatures" added to an online petition.
I remember asking people at Valve when HL came out if they were going to be doing an Amiga version (several people at Valve are ex-Amigans) and the response I got was that Valve only makes games for the IBM PC/clone* market.
In anycase, in order to get a company like Valve (a big time high-end games company) to be interested in the Amiga is for there to be actual modern Amiga's being sold to people (lots of people), and for the new Amiga's to be able to perform well enough to make their games look good. The only thing holding up progress is the fact that AOS4 is not out yet. Whether or not the AOS4/AmigaOne combo will be able to handle a game like HL (or more importantly, HL2) is unknown to me. Can someone tell me if it can?
*- Or plain old "PC", if you like.