I have used Instamapper last year and yes it dropped out when phone call received. Does Instamapper work on iphone4 in multitasking. I will try it .
I have used Motion-X on my iphone for 2 years now and I know it runs in background on multi tasking, But not sure it will upload and allow people to track live or online. The only thing I can mention is that we upload each run leg to a Facebook group TA RNR (Tri-Anglia Round Norfolk Relay). Someone mentioned that there is intermitant signal on the route so this may cause a problem.
Just an idea chucking here.............
What about these marathon hire pieces they had some for Edinburgh maathon for runners to hire, trying to look if we can hire from somewhere.
I have found this application and it is £2.99 it looks the same as motion-x but i think it does live tracking i'll have a look at it and see how it works.....
http://www.mapmytracks.com/sports/running