It might be that there isn't the demand until 64-bit Windows ships? HP submitted a TPC-C result for a 4-way Opteron 248 system running 32-bit Windows & SQL-Server which was in retrospect a tad disapointing, only fractionally beating 3.0GHz Xeons (all be it the 4MB cache versions).
The lack of 8-way Opteron systems is a bit surprising, especially given the competition scales terribly (IBM Xeon systems TPC-C 4-way vs. 8-way).