When I have some time I want to do a gps overlay on some gopro sailing footage, so you can actually know how fast you go when you do what.
I looked at the iphone as well for a sailing instrument and actually think its ideal, it has a gps, camera, data connection and a lot of other sensors like a gyro.
Maybe also thinking about putting this into some sort of dashboard with a video overlay.
If you have an iphone 3gs or 4 this free app registers everything the sensors do: http://itunes.apple.com/us/app/xsensor/id345145166?mt=8
A speedometer like the speedpuck looks interesting but speed alone would not give enough information without having something to compare it with (like maybe your speed on this side of the course on the previous lap or something).