We've detailed how to use Dropbox to sync your home folder across platforms, but for people whose entire workflow revolves around their desktop, that's not necessarily enough. The Mac blogger at Not Very Correct describes how he syncs his desktop, as well.
On Macs
His method is something we've seen before. Essentially he's created a symlink to sync files and folders outside the My Dropbox folder. On a Mac, the command looks like:
open up terminal and run the following commands
mv desktop desktop.bak
ln -s /Users/username/Dropbox/ ./Desktop(change username for your Mac username)
Things may go screwy for a bit, but don't worry. Your next step is to reboot your Mac and login again.
You may then want to go to your Desktop.bak directory and copy the files you want back to Desktop.
On Windows
Not Very Correct's post only covers syncing your Mac desktop, but I think we can get to the bottom of this on Windows. UPDATE: Reader paddirn offers up this simple solution:
There is another way that I've gotten to work with Windows:
Open your User folder, right-click on the folder Desktop, select Properties, open the Location Tab, and click Move; now pick the new location (Dropbox folder) and click Apply.
It's like magic and it's worked great for me for awhile now.
Dropbox Sync Desktop Between Macs [Not very correct]