Jeff's spot on here. In fact the person running for the board position could win without a vote by him/herself nominating someone else who they know would bow out.
No matter how many people you say it'd take to disqualify a termed out member (my rochambeau joke), a small group of people could bypass the election process and take over a board position.
The more I thought about it overnight, the more I'm convinced that any form of forced term limits creates a gateway to an organized corruption attack. Can someone convince me it wouldn't?