Patrick- one of the criticisms of Proboards is that there's "no escape". I know that that is due to the way you assign ownership of content (which, while not my preferred method, is at least sane in my opinion). My biggest issue is that while it's well known on TAZ that you can't take your content with you, it's not the most easily accessible information. Have you considered adding a "moving out" section to the FAQ or similar?
You can export all of your data by logging in at www.proboards.com, going to the Account Settings tab, and then clicking the Export section. This will let you download every thread, post, PM, etc.