This stuff is obsolete. There is ethfoo in -current which does the same, and there is a patch which makes it work on 2.0, too. Note that this patch is obsolete, too, since the ethfoo stuff is now available from pkgsrc.
This is a somewhat preliminary attempt to port the tap driver to NetBSD. It is far from complete, it is buggy, but at least on my 1.5 system it seems to somehow work. There is a small TODO list at the end of the copyright section of if_tap.c.