I purchased Gish quite a while ago (my original installer was v1.05), and have played it to death, and love it!
However, I have both a desktop and a laptop; also, these have both Linux and Windows installed. Currently, I've installed the game under Windows on the desktop and Windows on the laptop; I also had it installed on my parent's machine for a while so my mum could play it.
I recently upgraded the desktop machine (new CPU, RAM, harddrives etc. - so this "hardware ID" I've heard mentioned elsewhere would most definitely now be different), have tried to re-install Gish, and can't. I suppose this means I also won't be able to install the Linux version on my laptop now that it's out, nor on my desktop once I get around to re-installing Linux on it.
I understand the need to fight piracy, that you're a small company, and all that... and completely respect it. If Gish was my blood, sweat and tears, I wouldn't want people ripping me off either (plus the game isn't overpriced to begin with, unlike a lot of titles). But what can someone who's genuinely paid do?