Every Facebook application developer faces a big choice early on in their design: whether to build the bulk of their apps’ canvas pages in FBML or iframes. While there are advantages and disadvantages to each approach, those that choose FBML often get support for platform features well before those that choose iframes.

Tonight, Facebook is adding support for some platform features for apps with iframe-based canvas pages. Specifically, Facebook is adding:

• XFBML support, which allows iframe-based apps to use FBML tags
• Cached friend lists and preload FQL data support in the JavaScript client library
• Support for preloading FQL queries

XFBML support is particularly important because it allows many FBML tags not previously available in iframes to now be run directly in the iframe:

You can now use FBML tags on iframe canvas pages with XFBML. Many tags, like fb:name and fb:profile-pic, you can incorporate directly into your iframe’s HTML. Other important tags, like fb:request-form, you can use within the fb:serverfbml tag, letting you incorporate multi-friend selectors and request forms right into your iframe.

Developers can find the updated PHP client library here.