I was actually pondering this on a long drive yesterday, how weird. I decided that while I like Pontiac, it should be killed and consolidated with Chevy and Saturn. They really only offer two unique vehicles, the G8 and the Vibe. G8 should turn into the Impala and the Vibe should either be moved to Chevy or killed in favor of the Saturn Astra. The rest of the lineup should be axed, and the most popular Pontiac-only options on the G6 should be offered in it's Chevy and Saturn counterparts (I.E. convertible, three panel sunroof).
Hummer should be killed, and the new H3 truck hybrid thingie offered in GMC (with different sheet metal). Don't kill the GMT-360 (Trailblazer/Envoy), update it and offer a model in either Chevrolet or GMC but kill it's unibody brand mate (Acadia or Traverse).
Kill Buick in the U.S.A. and move it's most popular model to the Chevrolet or Cadillac brand.
Chevy and GMC need some differentiation in their truck and SUV lineups. More exotic vehicles like the Avalanche should be GMC-only, make GMC the cool truck professional during the week, play on the weekend 'foreman's' brand. Chevy should sell the vanilla 'workin' man trucks. Kill GMC vans, Chevy only.
Saab, IMO, is GM's most exciting brand - but only because of recent major changes. The public hasn't caught on yet. Keep running it more as a separate entity and don't mess with it, let it ride a few more years and see if they take off. Saab could very well be the new Buick, but a more exciting Buick that still engages old people but also draws the young urban professional.