Could be the battery, could be a drain on the battery. I would not suggest just going out and replacing it, until you get the electrical system checked; it might be a waste of money.
Not sure where you live, but batteries in hot areas have a problem with the plates shorting out due to heat. Not just Delco, but all lead/acid batteries. Therefore, yes, they can just go dead for no apparent reason. However, they can also go dead due to a drain on the battery that is caused by another reason.
Optima makes a good battery, they do hold up to heat better than lead/acid, but even Optimas can go bad.
I would suggest a trip to the dealer, let them run a check (they have a nifty little machine from Midronics). If it is the battery, they will replace it, if not, they should be able to diagnose why it went dead and repair the issue. Then, if you want a Optima, get it and donate the new battery to a charity that restores cars/trucks for sale.
Just a note, batteries are usually one of the top ten warranty items for ANY manufacturer; not just GM/Hummer. I know someone on the Lexus customer assistance center phones, and she has stated over dinner, that their number one call for a tow is a dead battery. I would guess, GM is right up there with Lexus.