I don't think it would be very hard. Just have it share smf_members and the SMF cookie, no?
Not with mediawiki... Its evil, it likes to make its own user databse....
maybe someone can copy the principle to make an auth_smf.php extension for mediawiki?
What the heck, ask and you shall recive, I garuntee that it may or may not work. Otherwise setup is same to the PHPBB plugin to the point where all variables are $wgSMF... instead of $wgPHPBB... Also you can forget this one while setting it up, $wgPHPBB_User_GroupTB. I will not support this file due to the fact I have no need for a wiki right now but I just wanted a small challenge that was presented, not to mention it seems to create users in the wiki user db and auth from that instead of SMF(sorta).
*edit*
Removed a few debug lines, and tested changing passwords. Well if password is changed in SMF, then you cant get into the wiki for it does not update the password there, and I dunno from there.
*/edit*
*edit2*
This has been only tested against SMF 1.1 RC2. I do not expect the 1.0 series to work at all due to how the password is stored changed since then from what I uderstand.
*/edit2*