I will admit being a bit irritated at Pete Brierley's question, and wanted a while to stop being irritated to respond.
Three questions to ponder. Would God desire universal salvation? Can God accomplish it? Can human or demonic action frustrate it? Obviously, I have a strong opinion about each of these.
For all the preaching of "faith not works" I hear from Protestants, the work of accepting Christ is often valued over God's act in Christ which predicates salvation itself. I cringe when I hear of evangelists "winning souls" (or even "saving souls") as if souls were carnival prizes.
Put crassly, I don't really care if someone doesn't want to be saved: the divine will promises and acts towards consummation. Those who know and own their salvation (I believe) are able to enjoy it, and through it praise and love God. This makes evangelism a potent tool for the human commonweal and not simply "fire insurance."
To evoke Susan B. Anthony, "failure is not an option" and eternal outsiders (used as a general term: assimilationist's is just as repellent as eternal damnation) is partial and penultimate.