Bryan,
If your budget will allow, I'd suggest bringing the IPhone using roaming from AT&T, however arrange some kind of international plan with them before you leave (one that includes voice and text) to reduce the costs somewhat.
BEFORE you arrive in Europe, disable the data portion of the phone (ie: the portion that uses the cellular network) or you'll face a HUGE roaming bill when you return home. Be sure to check that data roaming is "OFF" periodically during the trip, as I've heard from a few people that it can mysteriously turn "On" again.
That will allow you to:
use voice sparingly as calls will be expensive, including incoming calls, so be sure to tell your contacts NOT to call you unless absolutely necessary and tell them to check the time difference!!!
text messages when required, which will be the least expensive but can add up if used extensively
Wi-Fi for Internet access at Hostels or other locations with free Wi-Fi.
Using the GPS when out touring will probably use the data portion, so I don't believe that will be an option unless you're willing to pay for it.
I wouldn't bother with unlocking the IPhone (something which is often called "jailbreak") as I've been told by those in the industry that if the firmware is updated, the phone will re-lock.
Most IPhones charge fairly quickly, so you could sit in the room for a few hours and read a book while it's charging, or ask the staff at the front desk to charge it. You could also try to get a private room (available at some Hostels) every few nights so you could leave the room while it's charging.
Cheers!