If I install Ubuntu 12.04 Beta 2 now, is it possible to upgrade to Ubuntu 12.04 Final when it is released without reinstalling everything from a disc?
When you choose "Beta 12.04", you tell your computer to use packages that are "tagged" 12.04. They call it "Beta" since they are still in the testing phase, but you use the exact same repository that will be flagged as final when the beta period is over. It will be a seamless transition.
You can also go from e.g. 11.10 to 12.04 without reinstalling from disk - everything can be handled by online upgrades. If you have an older version of Ubuntu currently installed, just follow the upgrade instructions.