Michael Shanks has been booked for a one off appearance in a Supernatural episode entitled 99 problems.
A quick note from Michael regarding the casting:
“Just got booked on an episode of Supernatural called ’99 Problems’- not a huge part. Will let you know more details as they come in.”
According to the Spoiler TV website, the episode is the 17th for the current season of the series (Season 5) with shooting due to commence on 21 January.