Holy crap, I was just going to make a post about this in the iPhone discussion thread.
I think this is mainly targetted at reducing the damage that tethering will do to their network, now that they are finally offering it for iPhone users. Though current users with an unlimited dataplan will be allowed to keep it for the current rate ($30/mo).
That said, the two different limited packages are 200mb for $15 and 2gb for $25. You get a text when you get close to the limit, and it's +$15 for another 200mb or +$10 for another 1gb.
I just looked up my usage over the past 14 months, and of them,5 times I was over 204800 kb (200mb) data, two those I was less 4000kb and 2000kb over. Though the other 3 I was quite a bit more over. My peak usage was actually last month, at 261,020kb, which ~= 254mb.
It seems like I would be pretty safe dropping down to the 200mb plan and simply paying for the extra data if I needed it (paying the extra $15/month would simply bump me back up to what I'm paying now). Jenny is currently paying for unlimited, though she's never used even close to 100mb. If I had been on the 200mb for $15/mo plan... I would have saved $60-90 over the past 14 months just for myself. Maybe more had I actually cared about my data usage. Add in another $210 for Jenny's dataplan as well.
It seems like a good thing for me.