I didn't upgrade because I loved the form factor of the 3GS. I still love it and may not even upgrade to the 5 when it comes out. I will probably keep the 3GS until it dies. I don't need a camera (I already have 2 good ones). Don't need faster/more memory. It has all the apps I need. And it feels better in the hand than the 4. It's a perfect phone for me.
Ditto, although the retina display and improved camera/flash are tempting. Let's see what the iPhone 5 looks like.