I'm still keeping my 3GS and not upgrading to the new iPhone 4. It does everything I want and I don't need all the extra stuff that the new one comes with. I won't even upgrade the OS when it comes out until I jailbreak comes out that isn't buggy. I've been very happy with my 3GS as it does everything I want it to do and everything that my BB did short of BBM which I didn't use much anyhow.
As far as the data caps go, Unless i buy a new contract or add another phone to my contract then I'm still unlimited data. I'm on WiFi at work at at home so I'm good there as far as data goes.