Dooooh! I had just listened to ep 86 yesterday about Dann telling us to save .config somewhere outside of the kernel source tree when recompiling. I DL'd 2.6.15 this morning to run on my CentOS Workstation here at work. All was well. I configured a brand new .config file. Then I realized I forgot an option.
Went back into /usr/src/redhat/linux and as soon as i realized I typed make mrprope, my index finger got heavy and landed on the dreaded "R" key...dooooh.....start over!

I weeped, I yelled, I learned (hard way)
worked out for the better anyway...I added .config support in the kernel....in case my index finger ever gets heavy again:)
Morale: SAVE YOUR .config FILE!!