I still have my first iPhone, my trusty old 4GB 2G. I got it back in 2007, right after the price drop when they were selling out of their inventory of them ($299 for an iPhone?!?!? What a great deal!) and used it as my main phone up until the beginning of this year when I finally upgraded to a white 32GB 3GS. I kept the 2G around as a touchpod and general experimentation iPhone. When the iPhone 4 came out, I got a black 32GB one and gave the 3GS to my mother. She was still on her 8GB 2G, and even today still tells me "This phone is so *fast*!"
In short, yes, I have kept one of my old iPhones. My 4GB 2G. I have it around now as primarily a touchpod, something to mess around with hacking on, and as a travel phone since it's unlocked.