Beware: integrating with FC turned out to be a surprisingly large undertaking, although lots of that effort was spent a) trying to assemble an understanding from the FC docs about how the thing actually works and b) figuring out how to handle all of the weird cases (and the common cases, for that matter).
The FC developer docs are not all that coherent. The wiki seems to cover most of what you'd need to know, but lots of the information is on pages only linked to from obscure other pages, and there's not a good big-picture technical explanation. Even worse, much of the wiki is locked down so you can't fix it.
And there are a bunch of special cases to handle.. what if a user is logged into a facebook-linked local account on your site but is logged in as the wrong facebook user? Or what if they're logged into a facebook-connected local account and not logged into facebook? If you make a mistake, Facebook doesn't usually provide a helpful error message, and their javascript is all compressed, so it's a pain to try to use firebug.
If anybody's interested, my site's at http://mushpot.net. If anybody's got questions about the implementation, feel free to post them on mushpot and I'll try to respond.