I'm running a vBulletin forum, and i know it happens on other vBulletin forums as well..but there are always dozens and dozens of Yahoo! Slurp Spiders......WHY?!?!

Are they sucking up bandwidth?

That's because Yahoo is indexing your pages, so people searching for them with Yahoo can actually find them. Of course, it "sucks" some of your bandwidth, but generally not that much.

If you really get a lot of hits from it however, it could be possible that you confused the bot by using a lot of URLs that look the same, but are not. For example, if you use the get method to maintain your sessions, your index page, though it is the same could look like this:

index.php?PHPSESSID=4a54c1f54e4e57b98ae4ea
index.php?PHPSESSID=e7a84f46e8a8e6a4f68ea4
...

To prevent this, make sure you only use cookies to manage you sessions, or to disable sessions completely when a search engine is visiting you. If you really don't want yahoo to index your site, you should be able to disable it in robots.txt.

I would not be really worried about it....I mean it can only last so long, and really If spiders are "sucking your bandwith" you will need a dedicated server soon anyways.

From what I understand, adding a sitemap component such as VBSEO should help save on bandwidth. Of course I've only read this and can't comment on whether or not it's true but it does make sense.