Funnily enough I did track a couple of other cycle tourers using the TrackMyTour iPhone app - and I was very impressed. I would certainly be tempted to use that (instead of, for example, my Wordpress blog) for my next tour.
One tricky thing is getting lots of people to use it. As hinted in my last post, I think it would require something like an endorsement/recommendation from the ACA before enough people started using it for it to gain traction.
A bigger issue is that the only mobile device supported is the iPhone/iPad, with (according to their FAQ) 'no immediate plans' to support other mobile platforms such as Android, Blackberry, Windows Mobile etc. So its not exactly widely accessible (weirdly the iPhone app seems to offer functionality that you can't get on the actual website, e.g. following people's tours).
I agree about getting more people to use the app through promotion by Adventure Cycling Association. The Track My Tour app was reviewed awhile back on the Adventure Cycling Blog, http://blog.adventurecycling.org/2010/02/tourist-20-trackmytour-iphone-app.html.
I agree that having more supported devices would be nice to get a larger amount of the bicycle touring population using the app.