The rules have changed once again!
Just a head’s up:
On March 11th 2011 there won’t be any more opportunity to add FBML apps to facebook fanpages any more, only IFRAME will be supported. Existing FBML apps will still be allowed, but new ones can’t be created after that date according to facebook.
So if you were putting off creating that facebook fanpage until now, don’t hesitate any longer.
UPDATE ON WHAT TO USE NOW:
The official Facebook created Static IFRAME app is the preferred choice in the absence of FBML tabs. There are other copycat IFRAME apps that look the same, but they may ask for more control or permissions than they actually require.