Think your running in circles with all this build/factory speculation. About to take the plunge on a new IPhone 3G, so I have been trying to keep up on this 3G disconnect issue. Based on what I've been reading (e.g. mostly flashy headlines) Chuqui's blog seems to make the most sense. The link is below, I quoted the relavant info, but he goes on about Apple's relationship with chip providers. If you go to the link, the relavant info is kind of hidden (e.g. no glaring headlines), just search the page for iphone.
Quote from <http://chuqui.typepad.com/>
What I was told was that 90% of the disconnects are initiated inside the phone, which would exonerate AT&T. Most of the disconnects are being generated by crashes in the driver code for the 3G chip, which comes from the chip vendor, not something Apple written and outside of Apple's direct control. Complicating this -- even though Apple is handing over "here is the bug, here is the fix, update the driver", the turnaround from the vendor on driver updates is on the order of 2-3 months. Said, um, lack of urgency not exactly making people inside the projects happy.