(note: Beacon functionality & privacy settings have changed since initial launch, see info on Facebook Press page here)
it worked pretty much as described: after i finished purchasing a few new t-shirts & a cool hoodie ("Stewart/Colbert 2008 !"), an ajax popup in the bottom right-hand corner of my browser notified me a Facebook story was going to be published. didn't get a screen capture of that since it disappeared after ~30-45 seconds (roughly). my browser also blocked another busted tees popup (tho i don't think that was related to Beacon; not sure)
upon moving over to Facebook, i noticed at the top of my newsfeed there was a notification telling me that a story was about to be posted from BustedTees that looked like this:
and when i clicked on the "See More" it expanded to this:
clicking on the 'learn more' link takes you to this Facebook Help Page on Actions from External Websites.
clicking on the 'edit settings' link from that dialog brought me to this Facebook Privacy Settings for External Websites screen (also accessible from the main Facebook Privacy Settings as noted at the top in the red box):
anyway, that's the basics. i'll probably write a little more about this soon. as i had previously expected, some folks are finding the opt-out method controversial: MoveOn is planning to challenge Facebook on the opt-out privacy settings for Beacon. bet that won't be the end of this story.
still, privacy issues aside i think this is one of the more interesting innovations Facebook has come up with, along with the News Feed in general. assuming Facebook can work out the user privacy issues & come up with a good monetization strategy, this is going to be fascinating stuff...