I bought a 30 gb ipod video from a friend of mine. The plastic was cracked really bad and in the process of taking the cover off (the plastic front) I cracked the LCD screen in half. After replacing every component, apart from motherboard and hard drive, I began getting the sad face. On google I found something that said that it is from there not being enough pressure on the hard drive, and to cut a credit card in quarters and place in behind the hard drive (between the hard drive and the silver/metal backing). And well, it actually fixed it, I've used it for a year and a half, and am just now bumping up to a 80gb ipod video, the older ipod never failed to start or get the sad face (or any other error icons) after that.
good luck man!