I see the attributes loop and pulse on animation controls at http://developer.boxee.tv/Animations - however, they don't seem to actually work.

Code:
<control type="image">
  <posx>0</posx>
  <posy>0</posy>
  <width>1280</width>
  <height>720</height>
  <texture>background_lvl2.png</texture>
<animation effect="fade" time="300" start="0" end="100" loop="true"
 reversible="true">WindowOpen</animation>
</control>
This should cause the background to fade in over and over, but only does it once. Same for pulse="true"
FYI, I'm on the windows version 0.9.23.15885