Yes support for this was first included in 4.5, although I don't know if it was extensively tested.
http://www.openbsd.org/cgi-bin/cvswe...node.h#rev1.11
http://www.openbsd.org/cgi-bin/cvswe...node.c#rev1.43
http://www.openbsd.org/cgi-bin/cvswe...sops.c#rev1.51
Here is the original proposal/patch from Wouter Godefroy:
http://marc.info/?l=openbsd-tech&m=122696368805152&w=2
OpenBSD's implementation of ext2 does not support the journaling features of ext3, but it should be able to mount them.