I still have all three iPhones and all three are activated and on iPhone plans. Even though I don't need three activated phones, I just like having three activated phones having internet access no matter where I am or which phone I take with me. One of them will be retired though and it will likely be the 3G. I think I'll use that one to upgrade to the iPhone 4 though all three of my plans are eligible. I don't think I'll ever use the upgrade on the original iPhone because I like the $73 monthly bill on that one.
I'm going to keep it around. You never know what can happen to your new phone. My 3G will be a nice back-up plan. There's nothing worse than loosing/breaking your phone while you're still be charged monthly.
I am going to give my 3G to my wife and put the 200mb plan on it. $30 a month for data for her was always too much, as she just won't use it that much. But now that it's $15 I will make that plunge. She says she doesn't want it, but I am sure she will be happy after a week, compared to her LG phone. (She doesn't like change... haha)
im keeping mine sold my 3g last year but im keeping the 3gs... with ios 4 the 3Gs will turn into a new iphone... plus i just exchange mine yesterday due to dust and crack in the back...ive put a bestskin ever screen protector right away so i can preserve it as long as possible... and yeh i will buy the new one as soon as its out!