Tampa councilmen walk out on atheist's invocation

Here's an item from Floriduh:

Three Tampa city council members walked out of chambers rather than listen to an atheist give the opening invocation.

Council members Kevin White, Mary Alvarez and Rose Ferlita left their seats Thursday rather than listen to Michael R. Harvey, a member of Atheists of Florida who had been invited by council member John Dingfelder to offer the invocation.

[Full Story]

This raises an interesting question. What, exactly, does an atheist invoke? Himself? the almighty Nobody? the memory of St. Madalyn of the Oil Drum?