Up until today's announcement (December 1, 2007) the Singleton format has had only Skullclamp banned. Three new cards have been added to the Singleton ban list (Effective December 19, 2007) .
Upheaval becomes banned for similar reasons to it being banned from Prismatic. When it is cast, it is almost a guaranteed win. This power level is not desirable in a primarily casual format so we decided to remove it.
Flash was restricted in Online Classic and banned in Prismatic during the September announcement. It has proven to be a problem in Singleton as well. Strong tutors have made this combo card too consistently powerful to remain in the Singleton format.
Umezawa's Jitte has proven itself to powerful in the Singleton format. Games that involve Jitte revolve too much around the card. All four of the banned cards in the Singleton format have been removed because they are too game deciding.
Diabolic Intent was banned for being a powerful tutor.
The other fourteen cards banned have one thing in common. They all have the Transmute mechanic. Transmute turns all fourteen of these cards into powerful tutor effects. The fourteen cards all have different other abilities but the common link of being Transmute cards means they are being removed from the format.
Before this announcement twenty-seven cards on the Prismatic banned list are tutor effects. We have consistently removed tutors from the Prismatic format. We don't want a 250-card format to be about finding particular cards consistently and that is why we have banned fifteen more tutor cards.