If you’ve got a custom Facebook page and used FBML (Facebook markup language, similar to HTML) to create it, just a quick note to let you know that starting March 11, FBML will still SUPPORT previously created FBML pages, but you will not be able to make any changes to your existing FBML pages or create any new ones.
After March 11, Facebook will switch entirely to iframes rather than FBML for custom pages. (If you don’t know what a custom page is, check out mine at http://www.facebook.com/alohabeachcamp.)
Now if you have a custom page you’re happy with and don’t want to change it, you don’t have to do anything. But if you need to make any changes to your page or create new ones in the future you need to learn about Facebook’s iframes.