What is the minimum thickness of sheetrock/drywall required for residential construction?

In texas we use 1/2 " for res. and 5/8" when metal studs are used

3/8 for walls,1/2 for ceilings.

1/2 in for walls, and 5/8 on ceilings with 2' trusses, gives you better sag resistence. they also make a high strengh 1/2 drywall.

In Montana 1/2 sheetrock is the norm for walls and ceilings in residential construction even with 24" spaced Trusses. And 5/8 sheetrock for the inside of an attached garage, because of the need for added fire protection.
Commercial and institutional construction requires 5/8 fire code sheetrock in all applications for fire protection.
Don't forget to use 1/2" green waterboard or similar products for bathroom applications or where water or moisture may be a problem. This will prevent or inhibit the growth of mold.