It seems as the iPhone would generate significant data usage even when a preferred wi-fi network is available. How significant? Well, at least based on personal experience and on some scarce reports on forums, it could be as big as 20-30 MB at a time. Some other times it is only 2-3 MB or less, but it is always traffic occurring in the early morning hours, like between 2 and 4 AM, when I am pretty sure I would not use the phone (well, except for posting some drunk photos on Facebook Saturday night), and when I am sure as well that the home wi-fi connection would be available. The hypothesis is that the iPhone will not activate the wi-fi chip while in stand by (the screen is off), and would use the cellular chip instead, as that one is always on. This might have been designed by Apple with the unlimited data plan in mind and in order to increase the battery life. However, when we compare those phantom usages of sometimes 30 MB at a time to the 200 MB monthly limit on the new AT&T Data Plus plan, it sure looks like a lot.
You can check your own data usage by logging in to AT&T wireless and checking the past data usage by clicking on the "View past data usage" on the main page. (if you do not have an account, now it is time to set up one, as it will give you control over your usage and your bills including paying options)
What to do for now?
If you have the unlimited plan already, don't jump the bandwagon of the new plans... just yet. There is no going back to the unlimited plan once you do so. New features and applications (Netflix for iPhone anyone) might be data intensive and you might enjoy them better on an unlimited data plan. In all fairness though, if you only browse the web and do regular email on the iPhone, you might get by with the 200 MB plan... if your phantom night usage I have been talking about is not too high that is.
If you are becoming an iPhone/AT&T customer now, then you have no other choice but the tiered plans. You could start with the 200 MB plan, and watch your usage as you progress through your billing cycle, and switch to the 2GB plan in case you think you will be over 200 MB. AT&T confirmed through at least one of the customer service representatives that you can do that before your billing cycle is over, and you will not be charged for "overage" basically. In this way you could save $5 to $15 as compared to the $30 unlimited data plan, depending on your usage. You will probably have a different, more concerned attitude though towards your iPhone, which will become an other thing to worry about. And you probably should worry, as if you are not careful you could end up paying even more than $30 for relatively little monthly data usage. That is because while on the 200 MB plan, although you could pay as little as $15/month, you will pay an other $15 for going over your 200MB, and yet an other $15 if you consume an other 200 MB on top of the first 200 MB. You could pay $45 for a usage of anywhere between 401 and 499 MB. Similar story just higher limits with the 2GB plan. It is $10 for each 1GB increment on that one, so let's say you use 2.5 GB, you will pay $35 for data. Use 3.1 GB and you will pay $45.
Let's hope the iPhone glitch I mentioned here will be fixed in the latest and greatest iOS4. If not, let's make a big fuss until they will fix it.
