To everyone who is asking why our newly installed walls and ceilings are cracking, I have to say that we do not have a definitive answer. No, we do not live on a fault line!
After reading several "How to Sheetrock" books, it appears that fiberglass tape should NOT be used with pre-mixed joint compound. Even the Home Depot 1-2-3 book warns you that cracking may result. Special setting compounds are required for fiberglass tape.
Don't people use fiberglass tape and pre-mixed compound all the time ? Yes, but many times this leads to problems with joints cracking. The sheetrock god, US Gypsum, warns on their website, "Do NOT use fiberglass tape with joint compound, only with setting type compounds". This same warning is carried on their fiberglass tape.
Our contractor does not think this is the cause of the numerous ceiling and wall cracks.
If you are going to be installing sheetrock, I suggest reading more about paper joint tape vs fiberglass tape. Paper is significantly stronger and recommended by the majority of sheetrock professionals.