When I signed up for AT&T last June to get the iPhone 4, the only options I had for data were 200 MB or 2 GB; unlimited data was discontinued before I signed up. I opted for 2 GB to give me some room, in case I needed it. Up until now, I’ve used at most 250 MB per month. Since I work at home and mostly use Wi-Fi, I use most data on Wi-Fi. Furthermore, when I’m out, I don’t stream video or do anything that is high bandwidth. This month, I went on vacation (first real vacation in 5.5 years) and had no Wi-Fi coverage for the entire 5 day trip. My wife and I used the phone to find stuff to do, as well as for some Twitter checking, RSS feed reading, and email. It looks like I’ll use maybe 800 MB of my 2 GB allocation. So even with my high (for me) usage, I didn’t even use half of my allocation.
For many people, 2 GB of data is more than enough. If someone commuted on a bus or train and streamed music or video, they might use more data. So for social networking, web browsing and email 2 GB works and unlimited is just a marketing gimmick (at least it is for Sprint now). Of course, everyone’s usage varies, but for me, unlimited doesn’t mean anything to me. I suspect that when I add my wife to my account, 2 GB will be enough for her, as well.