AT&T says 65% of its smartphone customers use less than 200 MB a month, and 98% use less than 2 GB. [Source: http://www.usatoday.com/tech/news/2010-06-02-att-smartphone_N.htm]
With that in mind, and considering the data use of the 5 iPhones in my family, this move makes sense. 2 of the 5 would be fine with 200MB/month, and the other 3 (myself included) would be fine with 2GB/month...the highest usage any one of us has had from past usage history is about 1GB/month. And that's a lot of data for an iPhone, especially when you work on WiFi networks when available!
Yes, this move makes a lot a sense. It will save almost everybody money, except for the techies who visit sites like this, and 3G iPad owners. Hence all the negativity all around here. This lowers the cost of ownership for the iPhone by quite a bit though, so no doubt Apple is quite pleased.
As for me, I have Wifi at home and school, and I'm at either of those two places 95% of the time, so the 200MB should be just fine. Can't wait to get my first iPhone.