There are plenty of GPS speedo apps for the iPhone. That will be a good place to watch those to.
I have one installed, it's just called 'Speedometer' it lets you set 4 overspeed warning of your choosing and has a nice big digital display indicating actual speed. Many other features for checking fuel economy etc.
It also records a log of your journey which probably would be useless in a court of law but a nice feature
Cheers
Chris