My understanding from the speech is the 700 billion will be used to buy the illiquid securities. These are the securities that Bear Stearns cannot sell and many other financial companies (Vanguard, Fidelity, T Rowe Price et al) probably hold some of these too.
My further understanding is that 700 billion is being used because this is the value of the loans (at the times the loans were made). They are worth much less than 700 billion now. As the mortgage are repaid, the treasury is going to keep the payments to repay the 700 billion.
That is what I got out of the speech. Can someone tell me if this is too simplistic?