alias win7="bless -mount /Volumes/win7/ -legacy -setBoot -nextonly; shutdown -r now"
alias debian="bless -mount /Volumes/wheezy/ -legacy -setBoot -nextonly; shutdown -r now"
alias arch="bless -mount /Volumes/arch/ -legacy -setBoot -nextonly; shutdown -r now"
Obviously change the volume paths as needed. Typing "win7" into terminal will restart into my Windows 7 install, likewise for "debian" and "arch". Any subsequent reboot will automatically reboot back into OSX
Not quite your wish, but I recently hacked up another solution using an Applescript-based approach. Unlike other versions I'd found, this approach can be made passwordless. See the gist below and the first comment that describes passwordless operation:
https://gist.github.com/jwhitley/8377268
This can be invoked effortlessly from tools like LaunchBar or FastScripts.
Credit to @robjwells, whose original gist I forked and modified to be passwordless.
You could probably write a simple automator "service" that calls a bash script or something, and have it show up in the global services menu. You could define a key shortcut for it too.