Well, mint.com has a "???" filled in. They'll make money from referral kickbacks for the services they recommend. Hence why I was expecting to hear about some revenue generated.
Yeah... their affiliate programs pay insane kickbacks, ING Direct is around $100 dollars per new account with an additional $50 bonus to the person that opens a new account and they offer the best savings account rate in the USA, so Mint should get lots of signups.
ING Direct pays out less than HSBC, and I'm sure a host of other e-savings account that spend less to advertise. That said, I think you're right about them having a lucrative referral program. When I spoke with Patzer about it last summer, they were one of the big ones he mentioned.
idea -> code something -> market it -> users -> ???? -> $$money
so they are at the "users" part, once the figure out the "???" then they make some money.