Add this lines to your ~/.gitconfig to use Kaleidoscope as the default Git difftool & mergetool. Thanks to Nathan Cahill bit.ly/Lj0j6s

Kaleidoscope Git integration (AppStore version)

[difftool "Kaleidoscope"]
  cmd = ksdiff --partial-changeset --relative-path \"$MERGED\" -- \"$LOCAL\" \"$REMOTE\"
[diff]
  tool = Kaleidoscope
[difftool]
  prompt = false

[mergetool "Kaleidoscope"]
  cmd = ksdiff --merge --output \"$MERGED\" --base \"$BASE\" -- \"$LOCAL\" --snapshot \"$REMOTE\" --snapshot
  trustExitCode = true
[mergetool]
  prompt = false
[merge]
  tool = Kaleidoscope