I went about it a little different way. A few months ago, I used my daughter's non-smartphone and inserted her SIM into my original iPhone because she wanted to buy an iPhone 4. Activated it just fine with the original unlimited data plan. A little later plugged it into my iPhone 3G and everything transferred fine. Couple of days later she bought her iPhone 4 and when we plugged it in to active, everything transferred fine.
Two days ago, my wife left her non-smartphone in the rain and trashed it. Put her SIM into the original phone and it actived just fine with the original unlimited plan. Next day I plugged it into the 3G, as that was the phone she was going to end up using. Everything transferred and it transferred the unlimited data. There was a small problem with changing over to the 3G unlimited, but a 2 minute trip to my local AT&T corporate store got the CSR to manually enter the iPhone 3G serial number and everything transferred fine. It has unlimited data as well.
We have had Family unlimited texting for quite some time, so that never changed. Moral of the story, it still works!