That's why we're excited to learn that WordPress has just launched two new mobile themes for WordPress.com blogs that will be automatically displayed when a compatible mobile phone attempts to access your site.
For smartphones with top notch mobile browsers, like the iPhone or Android devices, the theme will be a tweaked version of WPtouch, while other devices will sport a variation of the WordPress Mobile Edition.
From the sound of it, WPtouch is pretty spiffy and even includes custom header image scaling. WordPress also shares that, "visitors greeted by WPtouch will get easy access to posts, pages, and archives. They’ll get fancy AJAX commenting and post loading." As for phones running less sophisticated browsers, the mobile theme will help your site load faster and display as much relevant information about your blog as possible.
Given that the new mobile themes are default, you don't have to do anything to see the new changes made to your WordPress.com blog. However, if you'd like to disable the mobile themes, you can do so from your Dashboard under Appearance -> Extras.