I have recently had my engine out of the car and replaced the head gasket, clutch, flywheel, etc..
Well when I started the engine back up it wasn't pumping oil. I immeadiately shut it down, and stormed around the garage for awhile since I knew this meant a few more days before I could take the car out.
After pulling the engine out again and taking apart the bottom end of the engine I haven't found any "smoking gun" type of problems. Possibly the upper oil pan gasket failed. I have a little time since I sent the bottom end off to get rebuilt since I didn't feel like dealing with it myself and lack of equipment and skill.
Hopefully this isn't that common since getting to the oil pump was a pain.
On a better note I ended up taking the eta out instead and ended up getting 37mpg going around 75. I love that car on the long trips!