Change the default editor for files opened in the terminal? (e.g. set it to TextEdit/Coda/Textmate)

Most programs will check the $EDITOR environment variable, so you can set that to the path of TextEdit in your bashrc. Git will use this as well.

Add the following to your ~/.bashrc file:
export EDITOR="/Applications/TextEdit.app/Contents/MacOS/TextEdit"
or just type the following command into your Terminal:
echo "export EDITOR=\"/Applications/TextEdit.app/Contents/MacOS/TextEdit\"" >> ~/.bashrc

If you are using zsh, use ~/.zshrc instead of ~/.bashrc.

Use git config --global core.editor mate -w or git config --global core.editor open as @dmckee suggests in the comments.

Reference: http://git-scm.com/docs/git-config

For OS X and Sublime Text

Make subl available.

Put this in ~/.bash_profile

[[ -s ~/.bashrc ]] && source ~/.bashrc

Put this in ~/.bashrc

export EDITOR=subl