Just received an email from Apple Store Customer Support on my yet to be shipped 3.0GHz Mac Pro:
To Our Valued Apple Customer:
Apple is pleased to announce a price drop for the Mac Pro you recently
ordered. We have automatically adjusted your order to reflect the new price.
For up-to-date information on your order, please visit our Order Status
website at http://www.apple.com/orderstatus. Once your order is shipped, you
can also obtain tracking information on this site.
Thank you for your interest in Apple products.
Apple Store Customer Support
For my custom built to order Mac Pro, this means ~$80 cheaper. Not a larger percent of the overall cost, but still certainly appreciated.
Update: This explains it: MacNN: Apple drops X1900XT BTO price