An interesting post over on the Dragon Age Wiki shows that PC users may be able to purchase and download Mark of the Assassin now, instead of waiting until the 10/11 launch date for the new downloadable campaign. (Note: the Wiki link contains possible spoilers)
As of the time of this post, we were able to download the campaign and confirm that it works with our install, though all options to purchase it have been locked, and the campaign has been de-listed from the XBox Live store entirely. So it appears that a select and vigilant few will be playing the campaign due to a technical snafu over at Bioware. We'll be working on an impressions post and will let you know our thoughts soon.