So, the bottom line is the following.
I suggest you to try these tests:
1. Create a blank YAFFS filesystem under 2.6.18-git w/ my patch and
verify it works (that's what I did recently again :) ). That is,
mount, copy, unmount, diff.
I hope the flash chip you've got is _not_ with syndrome-based ECC,
otherwise ping me for a patch to current nand_base -- it's broken for
syndrome case.
2. Create a blank YAFFS filesystem under <2.6.17 *w/
CONFIG_YAFFS_DOES_ECC unset*. Reboot with 2.6.18-git w/ my patch and
verify it works.

Please report any problems,

Thanks,
   Vitaly