My suggestion? Dispense with the invocation.

What with all the flap about Rick Warren being scheduled to give the opening invocation for Obama's inauguration, I keep asking myself "Why is there a invocation in the first place?"

I'm not happy with the pick of Warren, of course, as I disagree with his position on Prop 8 (amongst other things), but I believe having a benediction is the problem. There is no religious leader who would not bother somebody's sensibilities. This is a simple instance that proves that the separation of church and state should be taken seriously. How not to alienate groups of people? Leave religion out of it, as it should be.

Here's your Consitution factoid for the day:

Article I, Section II:
"Before he enter on the Execution of his Office, he shall take the following Oath or Affirmation:--"I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States."

Nowhere does it say that one must say "so help me God". Please note, also, that one can choose to either affirm or swear and that the "swearing" is not upon anything, or even to God. Article I, Section II is a purely secular statement. Read it as many times as you will; you will not find any religious promises in it anywhere.

Here's some quotes that I think are worth reading, though not specifically about the inaugural:

"[N]o religious Test shall ever be required as a Qualification to any Office or public Trust under the United States." Article VI, Clause 3 of the Constitution.

(The object of Article VI, Clause 3 was) "to cut off forever every pretense of any alliance between church and state in the national Government." U.S. Supreme Court Justice Joseph Story, Commentaries on the Constitution of the United States (1833).

And now, some words from former Presidents:

"The United States has adventured upon a great and noble experiment . . . of total separation of Church and State. . . . The offices of the Government are open alike to all. No tithes are levied to support an established Hierarchy, nor is the fallible judgment of man set up as the sure and infallible creed of faith. . . . Such is the great experiment which we have tried, and . . . our system of free government would be imperfect without it."-John Tyler

"The United States of America have exhibited, perhaps, the first example of governments erected on the simple principles of nature. . . . [In] the formation of the American governments . . . it will never be pretended that any persons employed in that service had interviews with the gods, or were in any degree under the influence of heaven. . . . These governments were contrived merely by the use of reason and the senses."-John Adams

More quotes here.
Large repository of first and secondary source materials on the separation of Church and State here.

Painting note: "General George Washington Resigning his Commission" John Trumbull 1824
"The citizens of the United States . . . have the right to applaud themselves for having given to mankind examples of an enlarged and liberal policy worthy of imitation. All possess alike liberty of conscience. . . . [T]he Government of the United States, which gives to bigotry no sanction, to persecution no assistance, requires only that they who live under its protection should demean themselves as good citizens in giving it on all occasions their effectual support."-George Washington

Addendum: This wikipedia entry on the history of Presidential inaugurations is quite interesting. John Quincy Adams was sworn in on a book of U.S. law. That seems most appropriate to me.