I don't think this functionality has been added yet and to be honest on most forums it was a waste anyway and I always disabled it in vB... I would suggest having this as an add-on, in-fact I think any functionality beyond the standard Forum should be an add-on
I do use the calendar on my forums because it's a great way of reminding people of events that they might like to attend. If it was an add on that would be fine - just having the option of a calendar would be helpful. Lack of a calendar however isn't going to turn me away from this software!!
You're right, the forum calendar is the more underrated and underestimated feature on forums. On my designs site I used it extensively, as did my clients/members.
My forum is probably one of the very few that did use it as it was meant to be used. I would definitely want one.