Well, I didn't have any success with Messages on 10.6
First, I modified the SystemVersion.plist to make the installer think my system was updated to 10.7.3. Then I installed the Messages Beta and restarted. I then replaced Snow Leopard's iChat with the latest Lion version and attempted to start the application. No go. Just an error message pop-up asking to report the fault to Apple.
Total bummer and unfair...first no iCloud for 10.6 (whatever happened to 10.6.9?) and now this. If AirPlay is made available to Core 2 Duo systems it may just be time to take the plunge, whatever the cost in battery life.