From this article it sounds like some progress is being made for Rosetta and Snow Leopard for Lion. John Carter sends these comments, "I’ve seen talk on the Internet somewhere that VMWare Fusion does the best job of installing any version of OS X as a guest OS using any other version of OS X as the host. And this by more than just one very serious and credible poster. They ranted me on my preference of Parallels for Mac for that purpose, so I bow to them."