http://www.hardocp.com/article.html?art=NDE5
(note the "grease monkey" comment, about how hardware is better than a tweak!)
They took a P4, 3.06, a couple of Vapochills (no details) and OC'd the SOB to 4.2 (temp -25 C), at which point, the heatspreader seperated, and the core overheated and cracked.
Hum... heatspreader...