I'm having the same problem, but only for one of our devices, a 8330 with the latest 4.5 OS loaded on it. We have BES 4.1.6 and I checked his UserConfig row to make sure HTML email was enabled. It's also enabled on the device and we have rebooted it. I deleted Desktop[CMIME] and resent the service books. No luck
Edit: Nevermind, he just told me it's working now.