The "Performancing" folks just released a plugin for Firefox that provides a fully featured weblog editor, ala Flock. I'm trying it out now - it appears to talk to WordPress OK (well, if this made it to the blog, that is).
It appears to have a decent WYSIWYG editor, but the image embedding doodad doesn't seem to have an upload utility - so I think you have to manually upload an image and then paste the URL into the image widget. Not fatal, but an "easy" improvement to the plugin. (I know, "easy" is oversimplifying it, since the plugin is aimed at being platform agnostic, so you'd have to have special cases for WP, MT, Blogger, etc...)
Update: Well, let's see... the HTML it produces sucks badly. Editing a post appears to create a duplicate post (leaving the unedited original, and creating a new one with the edited version). Don't think I'll be using this one much...
Update 2: I was a little hard on the Performancing Firefox extension out of the gate. I wasn't happy with the WYSIWYG HTML, but switching to "raw code" mode gives me essentially the stock WordPress text editor, which is what I'm used to. Also, I must have missed the "Publish as Edit" button - which is displayed right next to the "Publish to:" button - I don't see how I could have missed that big button. Anyway, if this post was edited, then it works fine :-)
Yup. that worked. OK, so now I see it as one step more advanced than the stock WP bookmarklets - since it gives you access to the post history so you can easily edit older posts. But, in WordPress, once you've been logged on, all posts anywhere you see them on your blog have an "edit" link, so it's so easy to edit older posts anyway...