> mkdir undo-rebase
~ > cd !$
cd undo-rebase
~/undo-rebase > git init
Initialized empty Git repository in .git/
~/undo-rebase master > touch one
~/undo-rebase master > ga .
~/undo-rebase master > gc -m "one"
Created initial commit 3dfffad: one
 0 files changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 one
~/undo-rebase master > touch two
~/undo-rebase master > ga .
~/undo-rebase master > gc -m "two"
Created commit 0c36fd9: two
 0 files changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 two
~/undo-rebase master > touch three
~/undo-rebase master > ga .
~/undo-rebase master > gc -m "three"
Created commit 6c4a8e8: three
 0 files changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 three

~/undo-rebase master > gl
...
commit 6c4a8e844de2809b23710fe84080187d7c6c89a6
Author: Jack Dempsey <jack.dempsey@gmail.com>
Date:   Thu Jul 16 22:40:00 2009 -0400

    three

commit 0c36fd9b1cf3bbc4ad29de5a62e4114465c307b3
Author: Jack Dempsey <jack.dempsey@gmail.com>
Date:   Thu Jul 16 22:39:54 2009 -0400

    two

commit 3dfffad2749dea6a187a32f7dd20fa4c13db452b
Author: Jack Dempsey <jack.dempsey@gmail.com>
Date:   Thu Jul 16 22:39:46 2009 -0400

    one


~/undo-rebase master > git rebase -i head^^
...
pick 0c36fd9 two
pick 6c4a8e8 three

# Rebase 3dfffad..6c4a8e8 onto 3dfffad
#
# Commands:
#  pick = use commit
#  edit = use commit, but stop for amending
#  squash = use commit, but meld into previous commit
#
# If you remove a line here THAT COMMIT WILL BE LOST.
# However, if you remove everything, the rebase will be aborted.
#

~/undo-rebase master > gl
...
commit 39521f7ed570f31b778b4281072131e72ada4c82
Author: Jack Dempsey <jack.dempsey@gmail.com>
Date:   Thu Jul 16 22:39:54 2009 -0400

    two

commit e0014a06d12f966bf0d332a0e07cbd8c1ecff21c
Author: Jack Dempsey <jack.dempsey@gmail.com>
Date:   Thu Jul 16 22:40:00 2009 -0400

    three

commit 3dfffad2749dea6a187a32f7dd20fa4c13db452b
Author: Jack Dempsey <jack.dempsey@gmail.com>
Date:   Thu Jul 16 22:39:46 2009 -0400

    one

~/undo-rebase master > ZOMG wait wait wait

~/undo-rebase master > git reflog show master@{1}

6c4a8e8... master@{1}: commit: three
0c36fd9... master@{2}: commit: two
3dfffad... master@{3}: commit (initial): one


~/undo-rebase master > git reset 6c4a8e8 --hard
HEAD is now at 6c4a8e8 three


commit 6c4a8e844de2809b23710fe84080187d7c6c89a6
Author: Jack Dempsey <jack.dempsey@gmail.com>
Date:   Thu Jul 16 22:40:00 2009 -0400

    three

commit 0c36fd9b1cf3bbc4ad29de5a62e4114465c307b3
Author: Jack Dempsey <jack.dempsey@gmail.com>
Date:   Thu Jul 16 22:39:54 2009 -0400

    two

commit 3dfffad2749dea6a187a32f7dd20fa4c13db452b
Author: Jack Dempsey <jack.dempsey@gmail.com>
Date:   Thu Jul 16 22:39:46 2009 -0400

    one

Voila!