Just been watching the sky linked press conf this evening so the following is based on th e adlib replays to those questions:
The 700 billion isn't going into a black hole therefore it’s not necessarily a bill of 700 billion...
The cash is to "buy" the (currently) worthless debt, the majority of which IS secured insome form, eg on property of one form or another. (hence the reason its not worth much NOW).
In 30 years time (a typical US Treasury bond length) the likely hood is that those buildings will be worth significantly more than they are being bought for now. Thus it’s unlikely to cost the tax payer in the medium/long term term.
Further, Congress has required the risk of those assets not being worth more in x years time to be borne by the banks themselves.
iX.