I have an AT&T iPhone 4, after having had a 3. I'm grandfathered in to the unlimited data plan. We now have 5 iPhones in the family, under a family plan. Everyone else has a 2GB limit - there is an AT&T app they use to monitor that.
The only cycling apps I've found were made redundant when I got my Garmin 5. One iPhone feature that isn't obvious is the ability to 'subscribe' your calendar to your Training Peaks account, so that you can see your scheduled workouts inside the iPhone calendar, along with any other calendar entries you've entered directly. I use that more than the Training Peaks mobile app, although that works nicely as well. I also have my iPhone calendar ties to my work Exchange account, so I can see entries from all three calendars (and email) in one place. I also subscribe my wife's iPhone to my Training Peaks calendar, so she knows when I am riding and when I am depriving my stepdaughter of having a car by driving to work.
Remember, you aren't totally cool until you install Groupon.