Yes, under User CP select the "edit options" on the left menu, then under 'Thread Display Options' the third box "Number of Posts to Show Per Page"
FYI this is the general setup for most vBulletin forums so it will be the same (or very similar, unless disabled by the Admins) on most Forums you'll come across (phpBB forums are similar but slightly different).
40 works fine for me. Make sure after selecting it you scroll to the bottom and select 'save settings.' You'll also need to reload any pages you're already viewing. But it's working on mine.
This sucks. I tried uping the setting to 40 and tried using Firefox, on both MAC and PC and Safari and even on the Droid browser - none show more than 25 posts on a page. And yes, my settings are saved with 40 posts per page.
Oh, well, I will try to create a new account and see if that works.