You can choose from how query optimization works and why it’s so hard, or how relational engines and MapReduce can work together. Expect that neither will be about writing select queries!
http://www.sqlpass.org/summit/na2010/Voteforthe2010DavidDeWittKeynoteTopic.aspx
He’s a great speaker, and I’m pleased he is returning this year. That said, I’m still campaigning for Jeremy Clarkson to do one of the keynotes, that would be a nice change of pace!