I just noticed that in Safari 2, you can mark an RSS Item as 'read' by simply clicking on the summary of the article -- the text directly below the title of the RSS item.
By doing that, you don't have to reload the whole page to mark items as read.
[robg adds: This one took me a minute to figure out, as the cursor in Safari doesn't indicate that the text itself is a clickable item. But click on the text anyway, and you'll see that the orange 'new item' header turns into the gray 'has been read' header. A quick search of Safari's help didn't find this tip...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051016070037861