I noticed that on pages which add and remove IFRAMEs through scripting, an empty item will remain in Safari's Activity window for that IFRAME even though it's gone from the DOM and thought I better report it as it might be a small memory leak. I'm not completely sure whether this is just a display issue in Safari or a true WebKit problem.
I added a reduced test case at the above mentioned URL - it will add and remove 20 IFRAMEs after pressing the "Start" button.
(oops, I read the priorities explanation too late, sorry for mail spamming)
Created attachment 11619 [details]
I don't think this is a memory leak. The Activity window tracks every resource ever loaded by that page, including ones that are no longer in use. But that goes away once the page is closed.