It will not be hard to make such mod but I just can't think how system will deal preference of each member..
Say I have 100 members,
each members has an average of 10 keywords set ..
Now whenever a new post is made, I scan that posts with 10*100 = 1000 keywords and then .. not to forget,
send mail to those members whose keywords matches...
Now imagine, a forum with mere 10000 users (that is average size of active forums) then each have 10 keywords, that would make 100,000 Keywords to be scanned.. then again a mechanism to inform matches.. ofcourse , can be optimized to great extent, already I have ideas coming for coding structure but still not sure if helps..
However, It can be done that Forum admin makes a public list, say upto 100 keywords.. and whenver a post is made, users those subscribed to those keywords get notified and users can select upto 5 or 10 keyword..