I'm sure this has been asked and swered before but its not in the FAQ's..

Where are his parents? Did they die some horrible and exotic death? Are they just too rich living off Sheldon's gazillions to be bothered leaving their luge lessons in the Bahamas for 5 minutes to check up on what he is doing? Does Sheldon employ them as the janitor and tea lady at Sheldonsoft where they get paid so little they can't afford more than baked beans each night which they heat up over a candle stub as they sit on packing crates in their weathered shack? Are they "lost" on some island with a bunch of other crazies after a plane crash and have just discovered 'the hatch' with the sounds of the 'mamas and papas' emanating from it and are trying to make some sense of all the flash forwards/backwards and sideways? Did they find some wardrobe in an attic and are too busy fighting the ice-queen to come back and check on him???...?

Where for the love of pug are they???!!!!

cheers
Sean

The only thing we know for sure is that Grandpa's daughter was Sheldon's mother. It's also been implied that Sheldon's parents dead.

Just to follow up on what was said, here are a few links:

Sheldon's mother was gramp's daughter [sheldoncomics.com]

The storyline starting here and ending Dec 21st [sheldoncomics.com] implies Sheldon's parents died.