The other side has to be upgraded as well. RIM is performing said upgrades to the BIS servers this weekend. You will need to push your service books down to your device again once they are finished. That should give you HTML email on BIS.
If you are also on BES, that is a separate upgrade not currently included in 4.1.5.
I may be downgrading back to 4.2 anyway, Ever since the 4.5 install it's been extremely sluggish, taking 10 seconds to scroll from one icon to the next, I've attempted multiple battery pulls - it's just slow, gonna play with it some more tonight before I just give up