I just did some testing by refreshing the "http://www.facebook.com/deactivate.php page and I always got two same friends. Next I realized that it was displaying a picture where both of us were tagged. I had untagged myself from a bunch of other albums, so the other 3 friends picked were pretty much random.

So, its definitely giving high priority if you both are tagged in the same picture irrespective of the fact if you are really good friends with them or not. One of the friend tagged in a pic with me was added about 4-5 months ago, whereas my other friend I have know her since 2001, but on facebook since 2004/2005. 70% of my friends added are from the 2004 - 2007 time period.