If you've got the tools, time, and skill, you can make a really elegant acrylic stand for your iPhone 4. But for those of us who don't (that includes me) here's a simple and cheap solution you can buy at most stationary stores. I used a 2.2" x 3.6" acrylic holder for name cards. It's totally unmodified, I just flipped it face down and rest the iPhone on top. Try to find one with thick acrylic because the flap of acrylic that folds over to hold the name card is what holds the iPhone. It works best for iPhone 4 since it has squared edges, so you might have to modify it a little if you want to use it with iPhone 3G.
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