Here's a proposal for an improvement: on the page's history, (cur) and (last) are junked. Instead, every listed version has two checkboxes in front of it. To get the diff from version x to version y, you check the first box in front of x and the second box in front of y, then click on the diff button.
I think the interface is even cleaner with just a single column of checkboxes. (You have to check two.)
Current PhpWiki does this. For an example, see:
http://www.dairiki.org/HammondWiki/AllTimeHammondPopHits?action=PageHistory
(Try it with and without javascript to see two slightly different interfaces.)