WASHINGTON -- Like many fellow Democratic politicians, Sen. Barack Obama is no stranger to the pulpit. But in December, Obama will go where few progressive Democrats usually venture--to a large, conservative evangelical church that boasts a Sunday attendance of more than 20,000 people. Even more unusual is that he'll attend at the invitation of megachurch Pastor Rick Warren, evangelical icon and author of the popular Christian book "The Purpose-Driven Life." Aides to Obama say he will appear at Saddleback Church in Lake Forest, Calif., on Dec. 1, World AIDS Day. "Sen. Obama has a deep respect for Mr. Warren's commitment...