I want to install the 10.6 developer preview and I'm probably going to use it more than Leopard, now...would it be possible for me to upgrade it and use Time Machine to continue backups, and then downgrade to Leopard if I need to and still be able to restore from a Time Machine Backup?

The only problem I can see would be if any Apple app in 10.6 uses a different preferences style than in 10.5. I don't think it should be something to worry about.

might become an issue as new builds emerge

I don't believe updating to 10.6 is worth it as of now as there is not a populated list of 64-bit apps you can use....other than built in apple ones.