The TechReport reports that a chip problem limit the supply of quad-core Opterons:

Chipmakers refer to chip-level problems as errata. Errata are fairly common in microprocessors, though they vary in nature and severity. This particular erratum first became widely known when AMD attributed the delay of the 2.4GHz version of its Phenom desktop processor to the problem. Not much is known about the specifics of the erratum, but it is related to the translation lookaside buffer (TLB) in the processor's L3 cache. The erratum can cause a system hang with certain software workloads. The issue occurs very rarely, and thus was not caught by AMD's usual qualification testing.

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." Industry sources have suggested to TR that those deals are high-volume situations involving supercomputing clusters. Such customers may run workloads less likely to be affected by any workarounds for the erratum that reduce L3 cache performance, and those customers could potentially consume hundreds of thousands of CPUs. Our sources indicate, and the current availability picture would seem to confirm, that quad-core Opterons are not shipping to OEMs or the channel more generally.