You can do this. It's time consuming but it is done before (I've tried it).
1. Back up with Time Machine. Your current machine right now w/ Lion Installed.
2. Format/Install Snow Leopard
3. Use Migration Assistant to move over your user account and applications from Time Machine.
4. You'll notice that Migration Assistant moved over some applications which are Lion only such as Address Book, System Preferences, etc.
5. Reinstall Snow Leopard w/o Formatting. Just put the disk in; click Install
6. Congratulations, you have Snow Leopard again. Run updates and check all your applications are still running properly.