Retro is something that the US has over the Japanese. But is something that can only be used in limited amounts. I would say that sports cars (especially the mustang and its pony car brethren) is something that the big three can push the retro car feel. People even in thier twenties know the pony car look and prefer it.
But with sedans GM and the other big three need to look modern, classy and sporty. Three things that are tough to pull off.
Chrysler has probably done the best job at this. Largely because thier cars do a good job at striking a balance. The 300 does well, because it is not retro, but because it resembles a british luxury car.
I think Pontiac has made the right move with the Solstice. But I think for every right move they make, they make about three design moves which are wrong. This can especially be said with the Chevy division, which has been overall bland in its design cues. I can fault ford for much of the same.
Pontiac and Cadillac have done a better job with thier designs, but not nearly as well as the Chrysler.
Additionally the continued focus of GM on SUVs is a HUGE mistake. Gas prices will not be going down ANYTIME in the near future. If anything they will continue to go up as China and India become more competitive in the Oil market.
The problem is when Americans move out of SUVs they tend to move towards the Japanese as far as cars. It is not that the Japanese have a corner on design, but design is the only thing that can entice drivers to buy american cars in addition to comparitive reliability and pricing. Chrysler has exploited this to a significant degree of success. The Performance line (Dodge) and the semi-luxury line (Chrysler) have done very well while GM and Ford have floundered. They focus on well designed vehicles, instead of following the Japanese Pack. Proving that if you do your own thing well enough you can survive.
I think GM can do well by making using pontiac for ultra-modern designs and giving them the performance to back the cars up. I think they are doing what they need to do with Cadillacs. Chevy is hard division to crack, but I think it can be used as a division to blend modern and retro designs (the HHR is a step in the right direction). The bland designs of the Cobalt, Malibu, and Impala have hurt them. The reliability of the Malibu has helped. Also they need to dump either GMC or Chevy trucks. There should only be ONE division in GM making trucks and SUVs, it will be a shrinking market segment in the coming years. They need to focus on car design and SUVs need to be an afterthought, not the other way around as it is now. It was thier lack of focus with cars which has put GM and ford in the position they are in now. All the while Chrysler was putting millions of dollars in distinguishing themselves.