I recently traveled to Ireland and was able to charge my iPhone in the rental car using a standard US car adaptor, however it fit rather loosely, so I had to keep checking to be sure the charger was making a connection to the car's power outlet. It seemed as though the outlet was not deep enough to firmly accept the US adaptor, but it still worked.
I also had no problem charging in the hotels and B&Bs I stayed in using a power adaptor for the wall outlet. I used one by Kensington which includes retractable prongs for different types of outlets worldwide, as well as an interchangable USB adaptor so the use of Apple's power brick was unnecessary.
Regarding International rates and service plans, I also signed up for AT&T's World Traveler plan for $5.99 for the month, and cancelled the option when I returned home. Calls will be billed at $0.99 per minute. SMS messages will cost $0.50 each, as someone else mentioned, however AT&T has a $9.99/month international SMS text option which includes 100 text messages, then bills at $0.20 per text when you exceed 100 messages. AT&T failed to tell me about this option when I inquired about international packages before leaving on my trip, so I ended up with the $0.50/text deal. I spoke to an AT&T rep about other questions after returning, and that's when I discovered this text package. I was told that on my next bill I'd see the text package added and my SMS messages prorated for that period. I was pleased that they did that.
I have to say, AT&T's reps are woefully underinformed about international rates and plans for the iPhone. It's probably due to the newness of the phone, but it's been very frustrating speaking to several different reps and getting different answers from each of them. One of the them told me the Edge network would not be available in Ireland unless I signed up for a one-year agreement with a $25/month international data plan, which I declined. But I discovered that the network did in fact work for me, albeit with the $0.02 per kB fee. The jury is actually still out on that fee, because the last rep I spoke to said I wouldn't be charged for any data use because the iPhone comes with an unlimited data package, regardless of whether I'm in the States or overseas, or whether my account details show the usage as part of the unlimited data or the pay-per-use data. We'll see. Just be sure you've checked with them about all the possible fees and plans associated with all the different features of the iPhone, because AT&T keeps them separate, i.e. phone calls, SMS messages, and internet data. With that said, I found my iPhone indispensible on my trip and was really glad I'd gotten it.
Of course now I've got another issue in that iPhoto won't recognize the phone anymore, so I'm stuck with over 900 photos that I don't know what to do with! Sorry if this got off topic, but any suggestions would be great...
Dan