How about cutting you off when you reach the limit?
What happens if you are downloading some huge file (thinking it was over wifi but you lose the wifi connection so your phone helpfully starts downloading over 3G) If you're downloading a couple dozen megabytes a minute, how quickly does the carrier recognize you are over and send you the text? How much has the overage cost you by the time you get the text and stop what you're doing? With 4G it only gets worse, and it becomes easier to hit that 2GB limit.
The people who are knowledgeable about tech (i.e. pretty much everyone who reads El Reg, I suspect) don't really have to worry too much about this, we're smart enough to notice if our wifi icon is exchanged for 3G, and to check our monthly usage totals now and then if we suspect we may be at risk for coming near our monthly limit. But how in the hell do we educate our less technically inclined girlfriend or parents about this?
Don't get me wrong, this is an improvement over how things were, but isn't exactly a real fix to the issue. I would rather have the option of being cut off, or have some sort of automated throttling in place for a plan that's theoretically "unlimited" (thus no overages) but just gets slower when the network is congested and/or you've been too greedy with your data that month. And don't get me started on roaming, when my girlfriend gets her new iPhone I'm going to have to insist she leaves it at home when we travel overseas, otherwise she'll turn it on and it'll check her email for $100 or something crazy like that :)