Most people who believe in the necessity of baptism for salvation would make exceptions for those who can't be baptized but would if they could (ie., the thief on the cross, the person trapped on an island alone with a Bible). Others, like the Church of Christ, sometimes take more of a dispensationalist view as to how the thief was saved. It's difficult to get around the fact, though, that in Acts the crowd is told to repent and to be baptized "for the remission of sins". I've seen a few attempts, but they mostly seem to really argue for special pleading for an unusual interpretation of the Greek grammar. Couple that with the fact that every one of writers from the early church who gave a baptismal theology links it to salvation, and it definitely seems like the New Testament writers had that in mind.
