I've only managed to boot a kernel from an usb stick plugged into the guruplug and formatted with the fat filesystem (fat because the vanilla u-boot from plugcomputer.org doesn't contain ext2 support):
usb start
set bootargs 'console=ttyS0,115200 ro root=/dev/sdb1 rootdelay=10'
fatload usb 1:1 0x06400000 uimage-guruplug
bootm 0x06400000
Trying to create a fat partition on the microsd card, it isn't recognized by the u-boot plug. Then it actually should read
...
root=/dev/sdb2
...
fatload usb 0:1 0x06400000 uimage-guruplug
...
For that test I partitioned the first partition of the microsd card to be an 100MB fat partition and the second partition to be the ext3 holding the rootfs. Unfortunately the u-boot cannot read the fat on partition 1:
Marvell>> fatls usb 0:1
Device NOT ready
Request Sense returned 02 3A 00
** Can't read from device 0 **
** Unable to use usb 0:1 for fatls **
whereas the fatls on the external usb stick succeeds:
Marvell>> fatls usb 1:1
2688588 uimage-guruplug
7173509 uimage-guruplug.vmlinux
2 file(s), 0 dir(s)