My wife is going to be making a rather sudden trip to the US next week and I'd like to stay in touch with her. My brother has said she can use his old iPhone 3GS, but it isn't connected to any provider right now.

Since I've been away from the US for over 3 years, I don't really know what's going on in the mobile world these days. Can I do the Wal-Mart "straight talk" plan for just one month? Do you just buy the sim card right there at Wal-Mart and walk out with a working phone? Do they have the micro sims that will fit in an iPhone? Anything else I should be aware of? I just want to make this as painless as possible for my wife as she'll have a lot of other things on her mind.