I've heard lots of people mention that they have to remove their DOC to boot off a hard drive, but I've been able to boot with the chip in, without a problem. I'm using a DTC Data Technology ISA EIDE controller that I bought at Fry's for $29.99 (ouch).
I also attempted today to build a 2.3.99-pre6 kernel today, but couldn't get it working with the DOC driver source. Apparently, the register_blkdev function in 2.3.99 requires different parameters than in 2.2, and since I'm not a kernel programmer, I didn't get any further than there.
The DOC Linux manual (which is excellent) mentions that you have to flash the firmware on the DOC to keep the DOC and lilo from conflicting, and then reflash it once you're ready to boot DOC-only, but I didn't have to do this.
I just got the Iopener LEM image booting successfully off of the DOC without hard drive. It required building a 2.2.14 kernel with the DOC driver and a DEVFS patch. Now, I'm back to the board looking for info to get X working...