I had been using Firefox for my blogging needs, but decided to try Safari again since I heard that some of the editing tools now worked in Safari as well (and I don't like having to launch Firefox just to update a blog). It was working fine on my girlfriend's new MBP, but I couldn't upload photos with my iBook. Then I remembered that I had locked the cache folder (~/Library » Caches » Safari). I unlocked it (Get Info » uncheck the Locked checkbox), and uploading worked fine.
I thought this hint might be helpful to others, as some third-party utilities (Safari Enhancer that I know of, maybe others) offer an option to disable Safari's cache.

