Namita Gupta posted the following information late last week at developers.facebook.com:
We are also moving toward IFrames instead of FBML for both canvas applications and Page tabs. As a part of this process, we will be standardizing on a small set of core FBML tags that will work with both applications on Facebook and external Web pages via our JavaScript SDK, effectively eliminating the technical difference between developing an application on and off Facebook.com.I'm looking forward to the change because iFrames allows to do so much more than the static FBML pages. I’ll keep you posted for anything new coming out of Facebook about FBML, but in the mean time I would start getting cozy with the iFrames…
We will begin supporting IFrames for Page tabs in the next few months. Developers building canvas applications should start using IFrames immediately. By the end of this year, we will no longer allow new FBML applications to be created, so all new canvas applications and Page tabs will have to be based on IFrames and our JavaScript SDK. We will, however, continue to support existing implementations of the older authentication mechanism as well as FBML on Page tabs and applications.