I am BGG's official honey trap
This came up recently - so your not the only one eagerly wanting to know more: Questions on Secret Santa 2013
In which Octavian said the following:
The announcement thread will come out in October when sign-ups begin. It will be posted in the front page Announcements section, so keep an eye out there (it's good to check that section regularly anyways - there are plenty of cool events announced there throughout the year!)
The expectation is that you will send new games from your target's wishlist with a total cost (including shipping) of at least $50 USD.
Hope you can be involved! This is one of my favorite parts of BGG each year!