Just like with the PowerPC to x86 conversion, Apple will have an emulation layer when they ultimately add ARM support to OS X. It was all seamless to end users.

Just like with the PowerPC to x86 conversion, Apple will have an emulation layer when they ultimately add ARM support to OS X. It was all seamless to end users. Photoshop et al ran under emulation for years before they eventually made native apps. I wouldn't be surprised if Lion contained ARM support courtesy of iOS. The "Back to the Mac" theme hinted at that.