If you're running an online business I assume you have already designed your fan pages on Facebook. If you haven't I don't think it's too late to jump into the bandwagon and join everyone out there. Many companies like Intel or Nokia have their own design on this social network and it creates quite a nice experience for users instead of staring at the old blue default color of Facebook.
I ran across this site today with a cool resource, 520grid, to help you design better for Facebook fan pages.
As you know Facebook allows 520px width of their main content area to display your custom content. So if you're big into designing using CSS framework, you're in luck! This framework helps you accomplishing your work faster and you don't have to worry about all the layout calculation and cross browser issues.
Check it out, have fun and share what you have created here in the comment section below for others to see as inspiration.