yes, i have found the nulls in var/log/messages as well. unless you also noticed corruption in other files, I am not very sure this is nand corruption. though I don't have a good theory on what the problem might be.
the writeable filesystem is already mounted with the "sync" option, which dramatically reduces performance (we can do that cause performance of the fs isn't really a problem in this scenario), and reboot already does unmounting of all writable file systems. So its definitely not that.
That said, both the nand driver and the ubifs code are in flux, and can be expected to be buggy. The main reason I believe its not a nand corruption issue is cause I only see these nulls in /var/log/messages, never in any other file. So a more probable explanation would have to do with logger and how logging is done.