This is a lesson that I learned the hard way when using an iPhone overseas with a 200MB data plan. That's the same amount of data offered by the new AT&T $15 scheme. [crarko adds: Which I believe takes effect June 7th. This will obviously affect the iPad with 3G as well.]
You likely know that you'll need to avoid many high-bandwidth activities -- watching video, streaming music -- when you're using a cellular connection if you don't want to max out your plan. Here's a gotcha that isn't immediately evident: updating your apps.
Depending on what apps you have and how many you need to update, you could, as I did, end up consuming 30 or more MBs of data in one gulp. (And since the size of the downloads isn't shown on the App Store update screen, you're not easily able to find out how much bandwidth you're about to consume.)
Better to wait until you're on a WiFi connection to get the latest software updates. If you have the new 2 GB data plan instead, perhaps this will be less of an issue, but still worth tracking.
[crarko adds: Apple needs to do a better job of clearly listing the sizes of these items in the App Store app. I'm pretty sure there is currently a 20 MB size limit on individual (not cumulative) downloads over the cellular connection, but if you have a number of apps that's a lot of updates to add up over the course of a month.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2010060215085421