I honestly don't think they ever will. And if they do, it won't be FREE. I would like for iPhone users to be able to use their unlimited data plan any damn way they want.
Basically, I guess I'm a proponent of a tiered data plan; but offer tethering because it would just count toward your data plan.
Maybe :
Unlimited data : $60
2GB/Month Data : $30
512MB/Month Data : $15
256MB/Month Data : $10
Allow tethering on all levels, no extra charge. If you blow through your data plan, then you have to pay per MB, that would be your problem to make sure your data plan fits your needs.
My son's Phone, through the family plan on AT&T, gets unlimited data for $10/month. Sure he doesn't use nearly as much as an iPhone does, but without it we were paying $30/month for his data usages.
On that note, the family plans should be double the costs for the double data configurations. This means that if you have 2 iPhones, the unlimited data would be $120. (4GB/month for 2 iPhones for $60) BUT if you have 3 iPhone, or even 4 iPhones, the unlimited data would still be $120. Another option could be to price them at 1/2 the price of the previous data plan. 1 iPhone = $60; 2 iPhones = $60 + $30, 3 iPhones = $60 + $30 + $15, etc...
Whatever they do, adjust the data plans, and GIVE US TETHERING!