Now finally I created a perl script for reducing the default config to the set of modules that are currently actually loaded. Reduces kernel compilation times on a quad core machine from 56 minutes to 6 for a standard SLED kernel

Usage:
# cd /var/tmp/linux-2.6 | or wherever your git tree is located |
# gunzip </proc/config.gz >.config | to get the current configuration |
# make oldconfig | to add new options for current kernel |
# ~/linux-adaptconfig.pl >.config.new | to remove all not required options |
# mv .config.new .config | |
# make oldconfig | to be on the save side... |
# make -j5 | build, mother*beep*, build :-) |
Yes, it's a hack. No, it's certainly not perfect. But it might be exactly what you had been waiting for. I waited long enough to actually write it myself...