I think our issue is may not be a checksum. I wrote my own little recovery program which works independently of the boot and recovery partitions so i can patch either/or without fear of bricking. Then for some tests, I left the stock recovery alone and started poking the boot partition with a stick.
I have extracted the boot partition, extracted the ramdisk, repacked the ramdisk, and inserted it back into the boot partition. My MD5 for the partition is now different, but it has the same contents as original. The phone boots up fine. I copied the original boot partition to the phone, but I intentionally screwed up the sha1 hash in the header, and the phone boots up fine. I started changing some unused bytes in the header and after the ramdisk to see if they weer included in any checksum and still the phone boots up like it is supposed to.
When I use my recovery app to clobber the boot partition, I get sent into fastboot mode. So far, from that fastboot mode, nothing I try has gotten me anything good. I tried booting the stock kernel and ramdisk from there and depending on what I do, it either crashes and gives a little message in teh ugly fastboot-blue text that it crashed, or it hangs forever on the fastboot screen, or it boots up to the LG logo and hangs.