If you head over to http://bloggenbent.wordpress.com you will notice that I use a text box for announcement, little personal ads, and such.
I know real ads are forbidden on WordPress.com as it is today, so no need to tell me to read the FAQ ;) but would it be possible for you to create a little IFrame text box with limits so that those of us with other domains and servers could use the text box a little more creatively?
Then all your server load would be handling the little text box, while the users would have to handle the content of the text box on our own?
That way you would still retain control of the themes, and your own security, but we users would be a little more free in what we can put on our pages?
Just a thought. What do you think?