Wetpaint now offers a simple wiki content back-up solution for site administrators and creators. First, sign-in. Then navigate to the settings area and click the last tab labeled ‘Backup.’

The back-up service will provide you with a zip file of the HTML output of current version wiki pages only. Extract the zip file to your desired location. You should be able to view the contents of your wiki pages in a browser.
Please note that the larger your site is (many pages, long pages, etc) the longer the exporting process will take.