I posted about this back in October when I first found out about it:
http://www.cheapassgamer.com/forums/showthread.php?t=306673
Back then, it was only available through Amazon (Android) and Apple's App Store. Since then, they've also released it on the Android Market.
The games are pretty fun, but despite promises, we haven't heard anything new from the developers since then, no sales figures to see how close purchasers are to goal, no updates to their website either. Also, since Amazon released it as a Free App of the Day, I recall reading they weren't honoring those free downloads towards the unlocking of the 6th game.
In the time I've owned the app, I've see one update which, as far as I can tell, only changed the app's icon, that was from Amazon's Appstore. Last time I checked, Android Market didn't receive anything besides the initial version...maybe they've abandoned the program altogether, who knows?