We'll be running a "Pinball Pinball Pinball" format, as discussed on TiltForums.
http://tiltforums.com/t/pinball-pinball-pinball-tournament-report-and-format-discussion/590

Registration 7:00pm to 7:30pm
Tournament starts at 7:30pm sharp!
Match Play: 7:30pm to 9:30pm
Entry Fee: $10

Head-to-head continuous match play across a two-hour session. At the end of each game, the winner gets one point. The loser stays at the game unless they've lost twice in a row, in which case the winner stays. The person leaving the machine goes to the back of the queue, the first person in the queue joins the open game, and the next game is started.

At the end of the round, no more games may start. Person with the most wins is declared the winner, with ties broken by fewest losses.