Referral - generally gets you (if you refer some one) a 1 time bonus ... not a life long revenue stream. All phones are discounted by the providers
You're absolutely right. It may truly be a one-time referral, with Apple receiving revenue from the two-year contract. (At the end of the two-year contract, the customer may renew where Apple doesn't get anything off of the new one. On the other hand, the customer may elect to purchase a new iPhone for another two-year contract and sell their old one on eBay

)
Then again, that's all speculation. We don't know the inner workings of the deal to truly comment on it.
Cingular charges rediculously high price for the Data service ~ 50$ or more for unlimited. How many of them will be willing to have a 100% rise in their bill?
I believe it's $39.99 for unlimited data. The $49.99 plan is unlimited data plus 1,000 text messages.
We should keep two things in mind:
1) Cingular is about to be transformed into AT&T, which may choose to change its plans to something more competitive.
2) AT&T may have some kind of special data pricing for iPhone customers. Keep in mind, Yahoo! has already struck a deal to give iPhone customers a free Yahoo! Plus upgrade to get the IMAP service. There's a possibility that AT&T may have separate iPhone prices, much like the special Blackberry plans.
Again, we won't know anything until June. So hang on tight, guys.