Maybe a login hook could do the trick?

sudo defaults write com.apple.loginwindow LoginHook /path/to/script

(For more information see http://www.bombich.com/mactips/loginhooks.html and http://docs.info.apple.com/article.html?artnum=301446 )

Using AppleScript you could try something like this:
osascript -e 'tell application "Terminal" to launch' -e 'tell application "System Events" to set visible of some item of ( get processes whose name = "Terminal" ) to false'

Btw, you can 'reset' Terminal.app by deleting the Terminal.plist file:
rm "$HOME/Library/Preferences/com.apple.Terminal.plist"