iFrames are definitely here to stay - for the time being anyway! Although they are not as readily available for the 'non-coder' Fan Page owner to access like fbml code was, iFrames certainly offers a lot more than fbml could ever do - and hence the reason Facebook switched.
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