I would love to know this as well, considering I'm getting the 4 soon and my gf already has a 3GS that's loaded with tons of apps I would love to get transferred to mine when I get it.
My wife and I do this all the time. As Kash said, you just have to use the same iTunes user account. One caveat, whoever is the second person to download the APP will see a message asking if you want to purchase (just like buying original) when you say yes then will inform you that you've already purchased the APP, so there is no double charge.
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