I have always heard that the tapered thread pipe actually seals metal to metal. The "dope" or "teflon" is only there to act as a lubricant.
Unfortunately, that's not the case. See for example,
http://en.wikipedia.org/wiki/National_pipe_thread , which explains, "... a clearance remains between the crests and roots of the threads, resulting in a leakage around this spiral. This means that NPT fittings must be made leak free with the aid of thread seal tape or a thread sealant compound."
