Funny, I've been using SL for a month or two and I actually just happened upon this post via Google while searching for a way to do exactly this with Grub from any OS. The reason I started my search was a mention in the KDM documentation mentioning it was possible to change the "default" entry for Lilo's next boot when restarting. I figured somebody had already done it for Grub and I've been sort of surprised to not find it anywhere yet. I'll post back my results for editing the next boot in Windows, and I might even end up making a batch file/shutdown script that will prompt you for the OS you'd like to boot on your next restart that will edit the grub.conf for you. Hopefully somebody can put together a script for Linux as well, although like somebody mentioned its probably more of a KDE/Gnome function that of Sabayon or any specific distro.