Java on Mac OS X is a bit different from every other platform. Apple releases their own heavily modified (but blessed by Sun and now Oracle) JVM via their Software Update system.
It is true that by installing 10.7 it will wipe out any JVM you may have had installed on 10.6, but fortunately Apple decided to reverse course somewhat, and a JVM can be downloaded for 10.7. The first time you try and run some program that needs it, you should be prompted to download it.
Eventually Apple wants to do away with this and just let people use the OpenJDK JVM, but it was nowhere near ready by the time of Lion's release so Apple is in something of a holding pattern. I wouldn't expect too many, if any, updates to the Mac OS X JVM in the coming days, unless there's some major security issue found in the JDK and the OpenJDK people still haven't managed to pull their act together.