[Nick Harris]

This type of fix is MUCH better than relying on .htaccess (some hosts like Yahoo! don't even allow the use of .htaccess) so I'd like to see this idea expanded. Here is my two cents, hope it helps, it's worth reading to understand this issue better at least.

Basics: What you WANT fully SEO'd is...
#1 - Forum home
#2 - Individual forums
#3 - Individual threads

Those are the money pages on any forum, the rest is fluff to search engines. Dupicate url's ARE a problem both because they create duplicate content in Google's eyes (2 pages are identical: "?p=" and "?t=") but a more important issue is it DILUTES LINK VALUE. It really does...

example: someone finds one of your threads in Google, they like it, they reply and then they share the article with friends, maybe even using that social networking toolbar you have on every page. What's wrong with that right? LOTS. For starters when they post a reply and return to the thread they are NOT on the same thread Google sent them to. When someone replies they get sent back to the "?p=" version and not the "?t=" version Google sent them to so when they tell their friends and create backlinks... yup - useless since they're not aimed at your already indexed page.

Solution - find every link on the site and aim them all to one version only.
New Problem - some features require the ?p= version. You can remove those features or not make these changes.
Bigger problem - having two versions is a core part of how vBulletin works so future releases aren't likely to change all links to point at one version, if they did older sites may stop working. This is going to have to happen at the user level and no plugin can accomplish such a big change without .htaccess so it's going to take someone writting an extensive guide and debugging it as they go.

EDIT: to be more clear about features. example: when you reply to a topic you are redirected to see your post afterwards, if you were to change the ?p= to ?t= you would end up back at the thread instead, not your post... fun stuff to figure out huh?