Recently, I've noticed people registering and then making semi-relevant posts in dead topics. That, in and of itself, doesn't really cause me a whole lot of concern - if someone wants to revive a long dead topic, I couldn't care less. I certainly do it. That said, it does seem a little spammish, all the same. Only problem is... as of late, these posts don't include any out-bound links. Atleast not when you first visit them. If, however, you revisit these posts a few days after they're made, you'll find that a signature has been created - a signature with links to spam sites.
It's an interesting strategy. People see these posts, don't regard them with much suspicion since they don't include links, and... never check them out, again. I certainly don't usually recheck old posts unless new ones have been made in that same topic.