I can't see how wearing an Ipod is any more dangerous than me trying to run without one, when I'm tired. For example, last night I couldn't sleep. Today I am tired, fratchety, clumsy, grumpy, inarticulate and cannot think straight. Heaven forbid I go for a run tonight - and if I do, sticking my Ipod on will only help keep me awake.
Everyone knows when training, long runs are the work of the devil. I cannot think of nothing for 4 hours. Granted, I can't listen to an Ipod for 4 hours, but I need to have the option there.
By all means, fun runners running for the experience of 'doing London' keep your MP3 player at home and relish the experience. But those needing to focus for times, or relying on Power Songs to get them past 23 miles need all the help they can get.