HTC G1 running Android, and using Google
My Tracks software. Mine's actually a Google developers' phone, and so is unlocked, but it's identical to a T-Mobile G1; the same software runs on the Android phones that Vodafone are now selling, and on other Android phones as they become available.
The benefit of the Android platform for this sort of thing is that you only have to carry one device. The downside is that when running GPS tracking the battery life on the standard battery is about five hours, so I have a double capacity battery (twelve quid off
eBay) which I use for long rides. The other downside of Android is that it's reliant on the network for mapping; when we got lost on the Exmouth Exodus under the escarpment of the Mendips we didn't have any network coverage so we didn't have a map.
However, over all I'm very pleased with it. When cycling one needs to travel light, and to have one device which provides phone, still and movie camera, GPS, network browser, compass and a whole slew of other functions seem to me a good deal.