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Using a sand box to "play with" a new major release of the OS is a bad idea.
Between major releases the bundled applications (Mail, iCal, Address Book, etc.) will often change how they store user data. In addition, there are probably also changes in how System Preferences stores its data (new options, changes to how old options are recorded, etc.). Since both the original OS and the new OS will share user data in a sandbox scenario, any user data that is upgraded to the format that the new OS uses will be unreadable in the old OS.
The problem would be lessened if you do not use any of the built-in application, but even configuration settings made in System Preferences might not be backwards compatible.
Just as examples, I know iCal data from Leopard (10.5) is not backwards compatible with Tiger (10.4). And Tiger's Mail user data is not backwards compatible with Panther (10.3). I do not know if Snow Leopard (10.6) introduces incompatibilities with Leopard, but I would certainly not bet that there are none. I have heard that the Migration Assistant will refuse to migrate user data from installations of newer OS releases, probably because of this backwards compatibility issue.
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