As long as you're using a smartphone, you can keep it.
I got an unlimited plan with my 3GS in December of 2009 and it broke a couple months ago and I've been using a Motorola Backflip and when I went to check to see if I could upgrade to an iPhone 4 (not the 4S, I was just checking to see if it allowed me to keep everything the same) and it got to the part of picking a data plan, it gave me the option of unlimited. I called them and they said it will be the same way with the iPhone 4S preorder.
I use between 1gb-4gb of data a month, and that's just for basic use when I'm not around WiFi. A 2gb cap on data would be a deal breaker for me. I'll stay with whoever has an unlimited data plan, even if it's more expensive.