This change looks like it might actually save me some money.
I currently have an iPhone 4s and iPad, both on Unlimited. I pay $60/month total, and I do so because ZOMG I have Unlimited and you can't get it anymore and I might use it some day and it makes me so cool to have it and blah blah blah.
A month ago, while on vacation at Disney, my credit card number was hacked, so my card was cancelled, and I forgot it was the card I used for month-to-month on the iPad, and that caused the Unlimited plan to get cancelled. I practically panicked and ended up convincing them to keep letting me have that plan ("convincing" is a stretch - they basically just said "um, OK"). The relief was palpable.
Keep in mind that, in the two years (almost) that I've had the iPad and three and a half years I've had an iPhone, all on Unlimited (yes, they do let you transfer it, at least as of the 4s purchase), I've gone over 2GB in a month once on each device. By about 100mb. My total monthly use (both devices) has never exceeded 3GB, not once. And I use each device all I want, whenever I want. It's just that, when I'm not around WiFi, I tend to be driving, so it's not all that easy to eat up the GBs.
I'll wait for the iPad 3 announcement to be sure there's not some trickery involved, but... it would appear that I could get the 5GB plan on the iPhone, always tether the iPad when I felt like it, and still have room to double my data usage without ever going past the plan limits, all while saving $10/month.
Damn. I must be missing something.