In my experience:
Flashing green = stock mtd0 u-Boot
Flashing orange = stock NAND Pogo OS
Off = new mtd3 u-Boot and USB OS (Arch Linux ARM install/Debian/etc)
I tried putting Lenny on last night and got the same behavior as you; that is, flashing green, off, then flashing green (which means Debian failed and it rebooted), flashing orange (means I'm back to stock NAND OS). However, I was just running /sbin/reboot or hitting the Reset button; maybe tonight I'll try a full powercycle to see if that helps. I was wondering why everyone else seems to be able to boot Lenny and I wasn't, so I hope this is the key. My USB-serial adapter is on it's way too but I'm too impatient to wait for it so I'm flailing about blindly.