I used the following web site to help determine which plugs/sockets are used in the over 20 countries we plan to visit so we could get those adapters. http://users.telenet.be/
Although a lot of the countries we will visit have different sockets, the European "C" plug is compatible with most of them. The "C" doesn't work in the US or Japan, but that is okay since our normal plugs work there. The only adapters we think we will need are the "C" and the "I" that is used in Argentina. We could just buy simple adapters fairly cheap, but we bought the Apple World Travel Adapter Kit for ~$40. It has more adapters than we need, but felt more comfortable using the Apple products since we know the quality is great and they are made to work specifically with Apple products. http://store.apple.com/us/product/MB974ZM/B
We could probably get by with just one set of adapters, but we'll plan on two sets so we can charge both our phones at one time. No need to start friction on who gets to charge first. I'm sure there will be plenty of other opportunites for disagreement along the way :)
P.S. Since the iphones coverter is actually connected via an USB cord, you could also just plug that into any computer to charge the iPhones.