Well I installed the Wordpress contrib and then created a new directory under that (i.e. /opt/wordpress/forum) and installed SMF there. This was before had a chance to play with the
SME Site Maker Script. I think if I was to do it again, I would create a new directory in /opt for SMF using the previously mentioned script. I don't know that there is any issue with dropping it in an existing /opt directory (I'm sure that someone will let us know if that is bad), but it seems to me that there would be some advantage in having them separate if you wanted to make some changes without affecting your entire site...
hth,
Ryan