I’ve been in contact with the WordPress.com support team and thought I should bring it to everyone’s attention here: to follow the WordPress.com Terms of Service you need to make sure that when you modify your footer content you keep the attribution links intact.
To do this you need to include one link to the WordPress.com home page and one link to the page related to the theme you are using. This can be found by following the link in your theme’s default footer. Normally it directs you to the website of the theme author or to an information page for the theme itself. You can still add in extras specific to your site but these two links need to be present otherwise your blog will be at risk of suspension.
Have a look at my footer on this page to see how I have achieved this.