You didn't mention which version of OS X you have. In version 10.4, a command-line utility called textutil was introduced. It will convert back and forth among various formats. I know it works with rtf, html, txt and doc. There may be others.
To get details about your options, just type textutil and hit return. The very basic syntax for conversion, in your situation would be:
textutil -convert html /Path-to-file/filename.rtf
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