i don't really understand your problem? Can you not just save the downloaded file somewhere on the computer, in case you need to reinstall?
As far as I recall, every time Adobe releases an LR update (be it a major step from LR2 to LR3, or even a point release of LR3 like 3.4 to 3.5), it is an entirely new download that does not need any prior LR to install. Even the LR2>LR3 update- you just download the whole LR3 program and enter in your serial number (I think it detects if it is an upgrade or a full-license serial # and asks you to put in the serial for LR2 also if it is an upgrade serial).
The whole program is less than a 100MB download. If you really want a physical disk, then simply burn the downloaded .dmg file to a CD and store it somewhere.
Backing up your entire system is a different matter, and as Dale suggests you can clone your disk or use Time Machine, etc.
Ruahrc