As long as we are speculating, I'll throw in my 2¢:
Rosetta will not be introduced into Lion, nor any of its successor OS'.
Rosetta is not owned by Apple, but licensed from a third party company, Transitive Corporation. It is a good guess that Apple's contract with Transitive Corporation has come to its expiration. Since the company was aquired by IBM, it seems doubtful that IBM will relicense the code.
This position is reinforced by the recent action of Intuit to release a five year old version of its Quicken program for the Mac that will run on Lion. This company has a notorious history of dragging its development of the Mac versions of its software, even though its former CEO, William V. Cambell, is on the Board of Directors of Apple, (He was CEO when Steve Jobs returned to Apple and Jobs secretly showed him the prototype for the iMac in 1997 and convinced Intuit to keep updates of its Mac version of Quicken alive at that time. In return for such guarantee, EVERY iMac sold in those days included a copy of Quicken). So, there would be no reason for Intuit to recode and release Quicken 2007 for Mac if there was ANY hope of Rosetta's reintroduction.
Hence those who need to run PPC programs into the future have one solution and two workarounds: Run Snow Leopard in Parallels in Lion; dual-boot a partioned or external hard disk, if the CPU hardware will allow booting from SL; and finally, keep a Snow Leopard compatible machine and treat it with kid gloves into the future (and use Screen Sharing to allow Lion and Snow Leopard's use on the same display).
A better question is: will Apple release an update to Snow Leopard that will allow iCloud access? If they do, will it be before the shutdown of MobileMe, now scheduled for June 30th? This would, of course, hit two birds with one stone (Rosetta and iCloud)!