I thought I might elaborate on one point in my comments that better addresses one of your complaints:
"Since the conclusion of your argument is that we shouldn't accept metaphysical premises, I would hope that you wouldn't ask us to accept a metaphysical premise (otherwise you would be contradicting yourself)."
The same situation applies for me. If I make the premise that we are required to accept a metaphysical premise in order for the world to have any meaning, then we should all hope that one of my conclusions is not that metaphysical premises should be rejected. This was actually a problem that plagued logical positivists and one of the reasons why positivism was eventually rejected by the philosophical community. By not being able to reduce science to non-metaphysical premises and concepts, they eventually were forced to accept some metaphysical claims, but then could no longer support their anti-metaphysical stance. You see Luis, your positivist philosophy has been argued for over 40 years (1920's-1960's), but it perished with Rudolf Carnap (here, I'll help you in your reply: http://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=Rudolf+Carnap+positivism&btnG=Google+Search ).
As far as the contention that by starting with a metaphysical premise and conclude with the permission to use metaphysical premises is not a valid contention. The reason is that the conclusion is not that we must make metaphysical premises so that we can have ontological theories. The conclusion is that by having metaphysical premises we can obtain successful theories of prediction as well as explanation in our physical theories. Without metaphysical premises, we cannot in some cases obtain predictive and almost always explanative success in our physical theories. By arguing against this argument, you have avoided the challenges that post-modernism presents. this leads to rejecting 'approximately true' designations of science (i.e., scientific anti-realism) but worse you are ultimately forced to hold scientific explanations as mere human convention.
Warm regards, Harv