Just bought aSmartHUD+cam app for the iPhone 3G and was pleasantly surprised. It turns the phone into a GPS camera warning device, much like roadAngel, but costs only 59p! It needs a bit of tweaking if you are in a built-up area and can't be relied on 100% of the time as it can pick up cameras on adjasent roads. For me though, it's money well spent.
To add to that, you can put the phone on the dash and set it to reflect the readings of the windscreen, like on the new M5. Works perfectly at night.
Admittedly it's not a purpose-built device, but perfect for occasional use or if you find yourself in an unfamiliar area and not sure where the local tax collection devices are located.