Received the 3 hour in advance on-time message of my flight to RDU today. Get to the airport, now delayed 30 minutes. At the scheduled departure time, I got a message telling me that I'd be arriving on time, but hadn't left yet. Then the flight was delayed 30 more minutes.

I've recently discovered flightstats.com, and I've found that it's alerting service seems to work quite well. At least for the times I've tested it.