I had the same thing happen to me. I had upgraded to the latest version of the OS on my 8330, and still did not have the option to display my mail in HTML. I did however, have the capability prior to upgrading to the latest version, because I was already using an earlier 4.5 version of the OS. My latest upgrade just happened to coincide with a conversion to a new BES server for our company. After several frustrating maneuvers and posts on the forum, I contacted our new BES admin to find out what version they are using. The BES software must also be the latest version or the HTML feature will not work. I also found out we are not using a true BES; rather on our server we are running BlackBerry Professional which allows us to connect the devices much like the BES but without all the advanced features (one of which is HTML mail). Our administrator told me this was selected to save cost and supposedly not lose functionality.
I did find a couple third-party apps that works really good for displaying HTML mail. They are BBSmart, and Empower. They can be installed OTA or through the Desktop Manger. The only thing with is they are not free, but do have a trial download.
You can go to the following websites to check them out:
BBSMart - BBSmart - Software for your BlackBerry®
Empower - Empower HTML Email Viewer, Blackberry Threaded SMS and more
There might be others out there, but these are the ones I know of. If anyone knows of any free ones, I would like to know of them as well.