Turn off international data roaming and you can use it for phone calls (at 99 cents to $1.29 a minute) but it won't use data off the 3G network.
So you will have to use it on Wifi.
But be aware that a lot of apps. which use GPS won't work without a data connection, which can be Wifi. Well they will work but some of their features are based on determining your location by GPS and a data connection.
I usually look for places to stay which has Wifi. So I can use the iPhone at the lodging but usually when you're out and about, it's not that easy to find an open Wifi and I don't really check.
The iPad will be unlocked so it would be possible to buy a prepaid data SIM for it. Apple is trying to negotiate favorable deals overseas.
All we can hope is that if they get such deals in place, they will eventually offer iPhones which are unlocked. Then US iPhone users might be able to use their iPhones overseas for reasonable costs.