Found at http://hexperimental.com/content/coding-iphone-apps-textmate
I have not tested this myself, but this guy suggests installing TextMate as your select editor in xcode, and then changing the launch shortcut to use applescript to send keyboard shortcuts
Bundle (Bundles > Bundle Editor > Show Bundle editor), xcode , Run :
osascript -e 'tell application "Xcode"
activate
end tell
tell application "System Events"
key code 15 using {command down, shift down}
key code 15 using {command down}
end tell'