Apple's done a couple of <i>really</i> bad things here:<p>1) Developers who build without XCode 4.3.1 and iOS 5.1 SDK cannot leverage the retina display, even if they embed hi-res images in their app.<p>2) In order to use XCode 4.3.1 and iOS 5.1 SDK, you MUST be running OSX Lion. You can not install it on Snow Leopard. This is bad - Apple has always supported "up to two versions back" but now, for a minor version release, has abandoned support for an OS that is still 'current'. Mountain Lion isn't released yet, and hence Lion and Snow Leopard and the last two versions.<p>What this means is developers who need to update their apps have to first buy OSX Lion (many have been legitimately holding off because it's pretty crappy, overall). This is specially worse for developers who build OSX and iOS apps - upgrading an OS and expecting your apps to "just work" isn't practical. I was just waiting to skip to Mountain Lion directly, but this forces me to pay Apple, test on and use Lion for a few months even though I don't want to.<p>Forcing developers to upgrade their OS just to support a .1 SDK update is a cheap, non-classy move by Apple.
Or, really, just test with 5.1 and fix the frickin' bugs. My wife reports a number of her games have stopped working. Not sure whose fault it is - Apple is secretive and developers are lazy - but these transitions are always rocky for her.