So Is It a Bailout or Not?

I really don't understand exactly what it is the Fed did yesterday that got the Stock Markets all giddy (though I'm happy for anything that gives me even temporary relief that my life savings are not doomed). According to the Times

the central bank offered to let the biggest investment banks on Wall Street borrow up to $200 billion in Treasury securities in exchange for hard-to-sell mortgage-backed securities as collateral. And the Fed made clear that it was prepared to do more as needed.

If these CDO's are being accepted as collateral, doesn't that imply they are being effectively guaranteed? Isn't that a bail-out? If nothing bad happens to the loans, then nothing will come of it, so perhaps it's just balance-sheet relief. But if it should come to the point that the Fed effectively assumes these securities, then won't there be enormous pressure to bail out the actual borrowers themselves?

Paul Craig Roberts has proposed simply suspending the mark-to-market requirement for these assets, allowing firms to value them at, say, 85% of book value for a year or two until the market can sort out the actual valuations. This would seem to provide the balance-sheet relief the Fed's action seems intended to provide, without the harmful side-effects that a bailout entails. But then again Roberts is a far-right, conspiracy-theory-promoting kook, so he can't possibly be right.