If you're one of the lucky ones who already has a WM 6.1 device, are looking at buying one in the future or have - ahem - procured one in some other way, the Microsoft Mobile Team Blog offers instructions on how to customize the sliding panel homescreen.
It takes a bit of work with the same home.xml file found in other WM standard builds, as well as some registry tweaks, but nothing more sinister than that. The bad news is that, as yet, the most you can do is re-order the panels, adjust how alerts appear in panels, and play around with images. They do say that they'll be letting us know more about creating panels in the near future.
As always, before diving into the guts of your device, be sure to back up your information. Or don't, if you like working without a net.