its sloppy but i got it.

diff --brief --recursive OLDDIR NEWDIR | awk '/Only in NEWDIR/ {print $3 $4}; /differ/ {print $4}' | sed 's/:/\//' | sed 's/NEWDIR/./' | (cd NEWDIR && tar zcf diff.tar.gz)

ugly, but it works. no way to put a dev file in a tar archive, huh? also, it doesn't tar up symbolic links since diff just follows them.