Played around with #cal and some #html5 and #microformats today. By default cal seems to have basic support for #hCalendar. I tried to extend it a bit and also add http://schema.org/Event declarations.
I don't know what this will be actually good for or if it is useful at all. Right now just playing bit around. Still quite buggy, not finished yet and also much space for optimisations still.

How do you add semantic to your events?

Heres some #TypoScript:
# configure single event view
plugin.tx_cal_controller.view.event.event {
  # Event title is displayed as page title and outside of the event declaration, so add it as meta-tag
  title.dataWrap = <meta itemprop="name" content="|">
  # declare image itemprop
  image.stdWrap.dataWrap = |
  image.1.params = itemprop="image"
  # this declaration is done in template because always visible
  startdate.dataWrap = |
  # wrap in HTML5 time-tag
  enddate {
    dataWrap = |</time>
    prepend = TEXT
    prepend.dataWrap = <time datetime="|" itemprop="endDate" class="dtend">
    prepend.field = end
    prepend.strftime = %Y%m%dT%H%M00
  }
  # TODO this should be visible if enddate is not visible
  microformat_enddate = TEXT
  microformat_enddate {
    field = end
    strftime = %Y%m%dT%H%M00
    dataWrap = <meta itemprop="endDate" content="|">
  }
  starttime.dataWrap =  |
  endtime.dataWrap = -| Uhr
  # The location of the event, could be much more extended by tt_address data like geo coordinates, etc.
  location.dataWrap = <div class="cal_loc" itemprop="location" itemscope itemtype="http://schema.org/Place">%%%LOCATION%%%:&nbsp;<span itemprop="name" class="location">|</span></div>
  # only hCalendar
  category_link.dataWrap = <div class="cal_cat">%%%CATEGORY%%%:&nbsp;<span class="category">|</span></div>
  # should be class 'description', but need to assign 'summary' to the title because summary is required by hCalendar
  description.dataWrap = <div class="summary">|</div>
  # MICROFORMAT_START does exist, also add MICROFORMAT_END
  microformat_end = TEXT
  microformat_end {
    field = end
    strftime = %Y%m%dT%H%M00
  }
}

In the event_model.tmpl I add a little bit:
<!--###TEMPLATE PHPICALENDAR EVENT### begin -->
<!--phpicalendar_event.tmpl-->
<div class="vevent" itemscope itemtype="http://schema.org/Event">
###TITLE###
###IMAGE###
###CATEGORY LINK###
###MICROFORMAT ENDDATE###
<time datetime="###MICROFORMAT START###" itemprop="startDate" class="dtstart">###STARTDATE###&lt;/time>###ENDDATE######STARTTIME######ENDTIME###
###LOCATION###
###DESCRIPTION###
</div>
<!--/phpicalendar_event.tmpl-->
<!--###TEMPLATE PHPICALENDAR EVENT### end -->