Dont know if anyone has posted about this yet, but looks like your SMF is using the addslashes() security feature. The only problem, is that the slashes arent striped when they are printed out, and theres also a better way to do this, you can use the htmlallentities() and mysql_real_escape() to fix this problem, if you want a full security function, i can give you guys mine to use.
For anyone who doesnt know what the addslashes() function does, here it is:
A normal " is displayed as /"
A normal &squot; is displayed as /'
It also affects other items as well
EDIT: Actually only some posts are this way, so this may have been fixed already, sorry if its a repost