I believe they call it "gravedigging" in forum land
the idea is if a topic isnt on the first page or two, then its fine to look it up under search but unless you have something major to add or announce about it, read it and let it rest in peace
Most people just have a problem with people bringing back from the dead a thread when they only reply "Great, that helps alot ... thanks" or just agreeing, but if it's relevant, and it's not too old, i don't think many people mind.
i'm with RedVB on this one
as long as its not spam, like jimboss said, but if they have another question relating to the topic why not? there's not much difference between posting there and making a new thread
i think it's pretty pointless when someone gives advice in a thread that is 2 years old. i'm guessing that the problem has been solved.
digging up joke threads with a "that is funny" is even worse.
there was one user digging up really old threads in the general section the other night and posting crap in them. i'm guessing he was bored. my tip - check out the arcade.