I know that what I'm about to say is a wistful dream, but I can hope that it comes true:
I hope they make the 10.7 update free of charge like iPhone updates. This would ensure that developers could be reasonably confident punters have the latest APIs making OSX a predictable platform to develop for (as opposed to Windows, where OSs dating back to the ark must be supported), that Apple does not have to keep multiple versions of OSX patched and that people who have not yet upgraded from Leopard can access the App Store (so Apple can take their 30% cut).
Not going to happen I know (I'll bet on a $49 price tag), but I can always hope.