RevdKathy inspired this idea for me:

t-mobile has an OK 3G coverage map here: http://www.t-mobile.com/coverage/pcc.aspx . It would be great if an app could once every 10 minutes or so check your location vs. the color of the 3g map and determine whether it should turn on 3g or not. This would be great for when I take long road trips. Of course, this is all assuming that it would save battery on the whole to check a coverage map via GPS rather than just check for 3G service.