You can do it automatically. We actually implemented something like that on one of our sites. I hadn't released the code, yet. But, just for you - here ya go:
http://wpmudevorg.wordpress.com/project/featured-blog-widget
Note that we created a customized theme so the widget displays in the main content area - not in a sidebar. I haven't tried it in a side bar, and it might not work as well there.
It seems like the screenshots I put on that site aren't working properly. Annoying. But, play with it - I think you'll get the idea.
We're using it in conjunction with the local_avatars plugin, since most of our users don't understand gravatars. The one caveat there is that local avatars has to be turned on in both the featured blog and the blog from which you're calling the widget.