The purpose of baptism

I've just returned from Oxford, where I spend a little time in the company of Doug Wilson, including hearing him speak at a talk addressing the misconceptions that surround the Federal Vision. These talks were rather helpful, and have given some food for thought, but one question that I want to mention, although not at this point answer, is the following:

If we believe that we are saved entirely through faith alone (which I do), what purpose is there in baptism?

That is to say, is it the case that we either accept the first clause (we are saved sola fides) and therefore could replace baptism by a public confession (or at least would simply have to retain baptism just to be obedient for obedience sake, which is not necessarily a bad concept), and the alternative is to reject sola fides and turn baptism into a ritual which either has to be completed as part of salvation or indeed brings salvation in and of itself.

It is easy to guess that I am beginning to feel that the above option do not comprise a complete set, and indeed that there are alternative ways of thinking, but as I said, I am not planning to answer my question now, I simply leave it as a place holder for future thinking. Be assured that although my fingers are not yet working on this topic, the cogs of my thoughts have begun to turn on the issue...