Sony announced this morning via its EU PlayStation Blog that the demo for Mafia 2 will be a "24-hour exclusive" to PlayStation Plus subscribers on the EU PSN (people presumably still reeling from their bonus copies of LittleBigPlanet). Non-subscribers in the EU will get the demo tomorrow. The demo went live via Automatic Download and on the PlayStation Plus section of PSN this morning.
If you can patiently outlast today, you'll find the demo available to every European PS3 owner tomorrow. It's also available worldwide on the Xbox Live Marketplace right now and should be added to the US PSN later today.