Hi Oren. It took a couple of weeks to create Yedda on Facebook. It took that much mainly because we experimented with various ways to implement Yedda while still retaining all of the Yedda functionality within Facebook (asking, answering, voting, etc).
We use a combination of FBML and IFRAME. Some of the pages like the application main page and the Invite page are pure FBML pages.
The rest of the pages are FBML pages which contains an IFRAME that contains the Yedda pages served directly from our servers.
This is being done by using the fb:iframe FBML tag.
There are slight adjustments due to various Facebook design limitations such as width, height and removing our tool bar at the top of the page, but all in all these are almost exactly the same pages you'll see if you'll go directly toe Yedda.
To sum it up, it was relatively easy to implement and achieve a good level of integration of Yedda within Facebook. If you have an idea for an application and a little bit of know how try it yourself. It's fun! 