On Multiply user pages with custom theme, this script adds a link to the right rail "View this page with Default Theme". Click on it to display the page using default theme.
Results may not always be identical to how the page should be displayed had default theme been actually used, but it should be close.
Update (2008-05-22): Now, the right rail entry added has two links: Default and Base. Click on Default to display the page using default ("clean") theme and no custom CSS. Click on Base to just remove the custom CSS, but retain the base theme.
Either link, when clicked, replaces the item by another link that reverts the page to it's original state.