Intel has discontinued and will stop offering completely
(PDF Press Release) the single-core processors rated at 3.80GHz and 3.60GHz. These are both single core processors, models P4 561 and P4 571. They've only been available for a short time, and though there are other similar processors that lack EM64T (571 and 561 support it), Intel has not said they will be discontinuing those.
At the current pricing, the 561 and 571 models are more expensive than the 600 series cpus, and are in some cases in the price range of dual-core variants. Most likely we will be seeing similar clockspeeds being offered in dual-core CPUs within the next year, however.