I recently moved my forums to to a /forum directory, and added a main page. This seems to have completely messed up my page rank (went from about 5/10 to 0/10). What can I do? Just wait it out until the next update and hope things get better?

Google will assign a pagerank to your new main page at the next update, based on how many backlinks you have at that time. There's not much you can do until then - Just get as many incoming links as possible, and hope for an improvement when the time comes.

Make a 301 redirect from old to new url and in the next update you should be fine with the PR.

I can't really do that, because my new page (index.php) has the same name as my old one :\

It's fairly simple, Toolbar PR is of no real interest or use so don't worry about it.

Can you name the old forum under the same file name? If possible because that will solve it