Yes. Basically, the question is more regarding interoperability between InstallShield 12 project files and InstallShield 2008 project files.
It should work in the same fashion as any previous release--when you open it with the newer version, it creates a backup with the schema designation as the extension (*.755 for IS12, IIRC), and then it upgrades the new copy that maintains the *.ism extension.
I've upgraded plenty of projects this way, myself.
