I thought this was well-put:
The primary reason why it's not such an oath is that it's spoken by children who are incapable of understanding and consenting to its terms. But that seems to me good grounds for objecting to the whole business. If the words spoken meant what they said, then they shouldn't be expected of children, and shouldn't have to be repeated over and over again. If they don't mean what they say, then they degrade language and the sense of solemnity that should accompany the swearing of loyalty oaths. If the words are serious, then they're inappropriate for the context (and 'under God' is a violation of the Establishment Clause). If the words are not serious-- and they're not, anymore-- if they're just mindless blather, then they demean something that shouldn't be demeaned.
It's interesting that some varieties of Christianity have limits on what counts as an endorsement of the religion-- what makes you a part of the club, as it were. You can't be born again without understanding what's at issue, so at least in theory you have to be of a certain level of sophistication; Amish teens take a year off before committing whole-hog to the lifestyle, and so on. But having first-graders say the pledge undermines the seriousness of all this.
The more I think about this, the more I agree that the whole business is tremendously weird, and the only reason it seems to make any sense is that we've all grown up with it.
Also, Balkin has a good old post here about ceremonial deism. It includes this interesting thought:
Congress might well have violated the Establishment Clause in 1954 when it added the words "under God," for Americans had gotten along quite well just saying "one nation, indivisible, with liberty and justice for all" for decades. But, over time, the use of the words has become comfortable like an old shoe, and has lost its religious edge. It's purely ceremonial, and we can retain it.