How can I "reboot into" the non-default operating system in a dual-boot configuration?

This tutorial solves this problem: http://www.webupd8.org/2010/10/how-to-reboot-in-windows-from-ubuntu.html

From the aforementioned tutorial:

3. Now to reboot Ubuntu in Windows or some other OS, run the following command: sudo grub-reboot X

where X is the menuentry position (starting with 0 as the first entry) of the OS you want to restart in from the GRUB menu. So if Windows is the 4th option in the GRUB menu, you would run "sudo grub-reboot 3". You can also use the exact menu entry instead of the menuentry position (like "Windows 7 (on /dev/sda1)") but entering just a number should be easier to remember. The grub-reboot will set the default boot entry for the next boot only.