A number of vendors (Keyspan, Belkin) make USB serial ports. FWIW, I use one here on this iMac and OS X with screen(1) and a null modem cable to act as a serial console for a headless Linux box.
+1 -Alex Goretoy http://www.goretoy.com Norman Mailer<http://www.brainyquote.com/quotes/authors/n/norman_mailer.html> - "Writing books is the closest men ever come to childbearing."
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