Edit: I'm using "revive", given the definition of "bump" in veinglory's post.
I think it depends on the thread and the forum. Sometimes a zombie thread is fun; sometimes it's rude. Sometimes, as Silver King just mentioned, it's more rude to start the conversation over again without reading the older one.
So let's look at rudeness a bit. That might make it clearer when starting new threads or reviving old ones becomes rude behavior.
1) It's rude to ignore what other people have said.
So if you're asking for advice, it's rude to start a thread without searching and reading old threads to see if it's already been answered.
If an old thread has some of the answer but not all, I don't think it would be rude to revive it and add another question. If you can't find any related threads, it's rude to revive an unrelated one to change the subject, but not rude to start a new thread.
2) It's rude to demand that people tell you things that don't benefit them.
Apologizing for the bother doesn't make it less rude. So if you're asking for advice, it's rude to ask again and again in different places if you don't get the answer you want. It's rude to argue with the answer and say "That's not what I meant" -- one should start the thread carefully, trying to ask what one really means to ask. And if nobody answers, that ought to be okay. It's nobody's job to answer.
3) It's rude to have your fun at the expense of other people.
So probably in Office Party or other fun forums, it'd be rude to revive an old thread that everyone already had fun with to have your fun and bore them with it. It's less rude to revive serious, writing-related old threads than old fun threads. However, it's rude to revive an old writing-related thread without asking a new question or offering a new insight -- that's wasting people's time.
4) It's rude to not respect other people's time.
So if you ask for advice and get it, you should take that advice rather than just asking for more, whether you're starting a new thread or reviving another.
Does that help?