In order to see HTML rendered e-mail on a BlackBerry simulator, no matter the model, the simulator must run off of a BES connection (corporate BlackBerry server), as the included ESS (the software POP/SMTP proxy app that allows you to test BlackBerry e-mail services locally), does not support HTML e-mail.
Alternatively, if you have no access to BES, you can still test HTML e-mail rendering with BIS (personal internet connection), but you would need to do so on a real device. You can build a program fairly quickly which listens to incoming e-mail on the device an then delivers the original source of the incoming e-mail to you for debugging.
That being said, the older models of BlackBerry (around RIM OS 4.1), do not support HTML e-mail. If they receive HTML e-mail, they will display the full HTML source code, tags and all. As of RIM OS 4.5, HTML support has been implemented in the BlackBerry e-mail application. In these cases, if the device receives an HTML message, it will attempt to display the HTML rendered format, as best as it can. If the device cannot render the HTML for whatever reason (such as if this is a simulator running with ESS), and the message is a MIME hybrid, where it contains both HTML and text-only parts in one message, the device will display the text-only version of the e-mail. If the e-mail message is HTML only, and doesn’t contain a text-only equivalent in the same e-mail, the device will attempt to strip out the HTML bits and tags and such, and will attempt to present the HTML message as it’s own text-only version.