Looks like the uSD slot and is supported booting from the shipping uBoot for the Guru does not yet cleanly boot from USB HD's or SD cards?
SO, that being said, I would speculate that:
1) You'll need a uBoot specific to the Guruplug that does support this, if what shipped from the factory does not have all the bells and whistles. Don't use the SheevaPlug version obviously either. You need to look into what newer versions of uBoot exist support uSD booting.
2) You'll need to configure uBoot to boot from an uSD card with the correct parameters, as I'd assume they will also be different from what many have documented with the Sheeva.
I don't have a great deal of time to spend on this currently (and don't have a GuruPlug to play with) - so that being said, spend some time in the PlugComputer.org forums and see what help you can get - its a great community. There are for sure guys who have gotten this licked. Check on the wiki, perhaps Marvell or others have updated this. Alot of geeks will have GuruPlugs starting to arrive now, so expect great progress on uBoot if it is in fact lacking with SD support.
When you get the info, Please come back and post here for sure and keep in contact. I can't offer much more value at this time, but i'll keep an eye out for further details myself in my travels.
WorkAround: IF you are waiting on Uboot SD support, it looks like USB flash disk booting is working, so you can image the disk image I have onto a USB flash disk instead, should work.
# /etc/fstab: static file system information.
#
# Use 'vol_id --uuid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# <file system> <mount point> <type> <options> <dump> <pass>
proc /proc proc defaults 0 0
UUID=5db75694-67aa-43bf-9ed1-17f220a66363 / ext2 errors=remount-ro 0 1
UUID=def092a1-2e3f-4036-8883-e5fda7569582 /boot ext2 defaults 0 2
Notice we mount based on volume UUID and not media specifics like /dev/sda or /dev/mmca etc

So as long as the device you image is as big or bigger then the 4 Gig image, and you image the USB Flash disk the same way and the same tools as the SD cards, should work.
*shrug*