Elmer,
Yes there have been demonstrations of samples and prototype systems that can't be benchmarked, but not production systems. You may not see the difference but the oems do.
Yes, the OEMs do know the difference: They have production Opteron processors in production boards, and I'm sure they have very reliable benchmark data.
By now, if there was a serious problem with Opteron, it could not be hidden. Rather, leaked benchmarks show that it scales very nicely and is more than competitive with Xeon. Soon enough those benchmarks will be official!
(Actually, Opteron looks competitive with Itanium 2 also, but that's a whole different market...)