# install git
apt-get install git-core
# clone Linus' tree:
git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git

# start bisecting:
cd linux-2.6
git bisect start
git bisect bad v2.6.21
git bisect good v2.6.20
cp /path/to/.config .

# start a round
make oldconfig
make
# install kernel, check whether it's good or bad, then:
git bisect [bad|good]
# start next round


After at about 10-15 reboots you'll have found the guilty commit
("...  is first bad commit").


More information on git bisecting:
  man git-bisect

----------------------
#other example:

# git clone --reference linux-2.6 git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-2.6.35.y.git
# cd linux-2.6.35.y
# git bisect start
# git bisect good v2.6.35.1
# git bisect bad v2.6.35.2