Hmm just had a quick look for you, and on the Webwiz site, looks like you can't. This isn't as bad as you might think, people clicking view all on huge threads can create quite a long and heavy load for the server, and it also would mean there would be duplicate content from an SEO point of view.
I'm still working on getting forum preferences integrated into peoples profiles screen, but perhaps as a good compromise I could see if I could have a setting to allow people to choose how many posts per page they see? At the moment it's 10, you could choose from 10-50 or something, would that be ok?