Is a defect in their processors the reason for the many delays AMD has seen with their quad core CPUs, such as their newer Opteron line and the upcoming Phenoms? According to some sources, CPU errata has been discovered by AMD that poses a problem for some of their customers. Due to a flaw in the processor, certain situations can cause it to hang. Apparently, this has affected AMD's willingness to ship it:

An industry source at a tier-two reseller told The Tech Report that the TLB erratum has led to a "stop ship" order on all Barcelona Opterons. When asked for comment, spokesman Phil Hughes said AMD is shipping Barcelona Opterons now, but only for "specific customer deals."
The only current fix, according to the article, is a very slow BIOS workaround. The problem with that, however, is that the performance hit to the system is severe, as high as 20% with AMD stating it is around 10%. With the only work-around besides fixing the design being a very slow BIOS fixup, this may very well be the reasoning for AMD's difficulties in getting mass quantities of quad-core to market.

If they have to make adjustments to their design this late in the game, it could seriously impact their marketshare as time goes on. Hopefully they will get this resolved soon.