Argos are selling this, the iLuv 1BA200. Its a bit more expensive than some others, but its the width of your average top tube, very light and keeps the iPhone running – so far about 6 hours for a 3G. 3GS would last longer, and 4 probably all day!
Looks a bit Heath Robinson I know, but it works and is simple in use. I used this on the Princes Risborough sportive. While the power was fine, the Phone lost GPS for about a third of the route. Big old fail. Oh well.
GPS does work up the Cotswolds and I have managed one great ride planned on Everytrail and followed in Cyclemeter. Slow though as I was recovering from the ITB: