It appears that "Under God" is perfectly allowable in the Pledge of Allegiance, as is "In God We Trust" appearing on US currency.

At least according to a San Fran Court of Appeals.

"The Pledge is constitutional," Judge Carlos Bea wrote for the majority in the 2-1 ruling. "The Pledge of Allegiance serves to unite our vast nation through the proud recitation of some of the ideals upon which our Republic was founded."

Unfortunately, that little blurb above doesn't say much in terms of WHY it's constitutional.  All I can figure is that it has to be because no specific religion or God is used.  Otherwise, it's anyone's guess why this court ruled in this manner.  We'll see what happens when/if this ever goes to the Supreme Court.