Okay so this only works in Boot Camp 3.0+ which comes with Snow Leopard (10.6). If you ever find yourself in Windows and need to quickly — using command line — change your boot option to reboot into OSX (process known under OSX as blessing) you can use the following command:c:\progra~1\Bootca~1\bootcamp.exe -StartupDisk "Mac OS" Shutdown /r /t 0
This will reboot your windows session and boot the machine back using the Volume named “Mac OS”. If you omitt -StartupDisk option, then the system will boot into the first Mac OS X volume located by BootCamp.exe. Done :-)