An ipo would bring no more of a premium to the value than a direct sell. the market will not pay a distressed company a premium whether in a public or private placement.
rumors floating that berkshire is interested in gmac but he will never pay a big enough premium for gm to sell imo.
if gm thinks they are going to get a big premium with an ipo they are dreaming. remember the markets know that gm is distressed that is what will dictate the market mood for the offering the same as it will for buffett to step in and pay just enough to get it off gm's hands.
if gm sells gmac to buffett for little to no premium they will be making a deal with the devil and end up in bankruptcy when it is all said and done.