There are tigh supplies of P4 LGA775 CPU's in Taiwan right now. The shortage isn't set to ease until Q1 next year.

In addition, some distributors in the channel said Intel has informed them that it could only deliver 30% of the mainstream CPUs they require for the first quarter of next year.

Some sources in the channel speculated that the shortage might be the result of Intel shifting some of its production capacity for desktop-use CPUs to the production of notebook-use CPUs and chipsets.
Digi Times has more.