I agree that the post page would be a much more visually and thematically obvious place to put it. It might cause some complications, since I doubt (but honestly don't know) you'd be able to refresh the display via AJAX, which would mean submitting the page and presumably any changes you made to the post to display the editing interface you'd picked. I'm sure you could handle that with a messagebox advisory saying 'Warning: this will submit any changes you've made to your post. Continue?'
I'd also, however, like to see a global setting under options. Something like an 'Allow users to use the visual rich editor when writing', with a second choice enabled to indicate 'Default: On' or 'Default: Off'.
Maybe it's just me - I assume tinymce has its fans, but to me it seems like the blog equivalent of using FrontPage.