Today you would have opened up your Facebook fanpage and been invited to update to the new design... make sure you read this and preview your new design facebook fan page BEFORE you decide to upgrade!
No need to panic though! Facebook will still support Fanpages already using FBML code. It will only affect Fanpages wanting to create a custom page from 2011. Here's an excerpt from Facebook Developers list of changes expected in the first quarter for 2011