Just for grins, I decided to pull most of the file systems out of my 
kernel and start using the lkm versions.  Most of them seem to work just 
fine, with the exception of union.  Here's what happens...  (Note that 
when the two test directories, test/one and test/two, are mounted in a 
union, only the "upper" file system's contents is visible.)

	+ mkdir test
	+ cd test
	+ mkdir one
	+ touch one/file.1
	+ mkdir two
	+ touch two/file.2
	+ ls -l one two
	one:
	total 0
	-rw-rw-r--  1 paul  staff  0 Nov 15 05:06 file.1

	two:
	total 0
	-rw-rw-r--  1 paul  staff  0 Nov 15 05:06 file.2
	+ mount -t union ./one ./two
	+ ls -l one two
	one:
	total 0
	-rw-rw-r--  1 paul  staff  0 Nov 15 05:06 file.1

	two:
	total 0
	-rw-rw-r--  1 paul  staff  0 Nov 15 05:06 file.1
	+ umount ./two
	+ mount -t union -o,-b ./one ./two
	+ ls -l one two
	one:
	total 0
	-rw-rw-r--  1 paul  staff  0 Nov 15 05:06 file.1

	two:
	total 0
	-rw-rw-r--  1 paul  staff  0 Nov 15 05:06 file.2
	+ umount ./two
	+ cd ..
	+ rm -r test