I just noticed that WordPress.com now enables you to add a little more customisation and branding using “blavatars” or blog avatars (I guess this is built out of Automattic’s acquisition of the Gravatar service).
If you go to the Dashboard -> Settings -> Blog Image you can upload an image which will act as your blog’s favicon:
A look at the page source also indicated that it would appear as an image on the iPhone / iPod Touch Home Screen:
A nice way to add a little more personalisation to your site. There’s more information on this on the WP.com blog.
(Images courtesy of LittleSnapper on OS X, and Sketches on the iPhone)