Just a quick note, iPhone OS 4.0 Beta 3 has been pulled form the Apple Developer site, this is because of “flash rom” overheating, I urge you
Do not upgrade to 4.0 beta 3.
Wait for Apple to reupload beta 3 and download as appropriate.
Seems all is not as bad as first thought, rumours were circulating that it was overheating, turns out it is a problem in the SDK. Sorry for the alarm guys.