This is why I went Wifi only this time around. I found I never used 3G on iPad because I never used it to its potential, so I got rid of my "unlimited" (quotes intentional) iPhone plan because I never used more than 2GB in any month ever. Now I have 5GB with tethering between 2 devices for $20 more a month. I'm never without my iPhone, but I'm often without my iPad. 3G speeds are fine for me.
I'm the opposite. I'm on my iPad all day long and my iPhone sits in the charger waiting for me to pick it up when I go out (I'm retired). I have unlimited data on both my iPad and my iPhone with AT&T. I just bought a iPad 3 64GB with data. I had the unlimited plan with my other ipads and I'm gonna keep it with the new device.