I've had two SIM cards that died suddenly with no apparent cause (the second was last week in Iceland in a remote area rather devoid of dwellings or traffic, with just lava fields and some recent ash). A stark reminder not to rely only on your cell phone for emergency situations.
I find an iPhone useful for browsing once you're settled for the day, but find it too fragile to mount it on a bike and expect it to be functional after a crash (when you might need more).
Cyclemeter is a great app for cyclists, otherwise. As a GPS, I still prefer an eTrex (more rugged, longer battery life). iPhone's camera sucks (I have the 3GS model), for anything more than "utility snaps" (shooting a snippet of a map or some directions something like that) I prefer to use a dedicated small camera such as the Canon S90.