Um, are you sure about Exchange 5.5? If Exchange 5.5 didn't support HTML messages, then how could you use Outlook Express, for example, to send an HTML message over Exchange?
My impression, having launched Exchange 5.5 and being involved in many of the perf tests, is that Exchange 5.5 *always* generates the RTF property when encountering an HTML message, and then stores the RTF alongside the HTML. This of course is a performance and storage bummer if none of your Internet-native clients understand RTF to begin with.
Exchange 2000 (and 2003) doesn't do any conversion from HTML to RTF at all, unless the message is specifically requested by an RTF client. From that point, the RTF bodypart is available on the message; no further conversion is necessary should other RTF clients request the same message.
Chris