the best app so far is racechrono which runs on Symbian (nokia). Not sure about the new version of Symbian but the old phones, like the E and N models works great. Need an external GPS receiver to pair it via bluetooth and it is smart enough to count the laps, lap time, top speed, etc.
Takes a snapshot every second (dont know if that can be changed) of your location and it calculates speed that way. best of all its free.
The developer was talking about making an Android adaptation but i wouldn't expect anything for a while. Dont think he can write this app for iPhone as Apple would probably reject it and not allow it on the market. Also iphones have a shit GPS receiver so even if it did work i wouldnt want to use it. Pairing an iphone to an external GPS receiver is impossible as Apple have also decided to block anything that isnt a headset or audio device to connect via bluetooth.
short answer, yes there are lap timers out there, ive seen and used 3-4 free and paid apps and compared to racechrono they are all rubbish.
Although if you want to get very fast lap times there is an app for iphones which clocked me doing 1:31 at EC
