Yes, you can use both. Ironically, a GM employee is NOT allowed to use both the employee discount AND the GM card earnings, but non-employees (like myself) can.
I just picked up a 2005 Colorado Crew Cab which stickered at $22,845.00 but with the employee discount and the current $2,000 customer cash it was $18,145.00! I then applied $1,000 worth of earnings from my Dad's GM MasterCard which brought it down to $17,145.00. After receiving $2,000 for my trade (a '93 Aerostar w/ 95k miles) and handing them $2,500 cash down, I financed just over $14,000.00 after tax/title/docs.
Another thing worth mentioning....they offered me the credit life insurance, which pays off your loan in the event of your untimely demise, and normally I would stay away from crap like that....but by opting FOR it, they lowered my interest rate a full percentage point which actually lowered my monthly payment and will save my over $500.00 in finance charges over the life of the loan. The premium for the policy, incidentally, is only $350.00.
So...anyone on the fence right now about purchasing a new GM vehicle ought to go looking at a few before July 5th. Just stay away from the more problematic models (i.e. anything with the 3.4ltr V6) and you should be good to go!