I found something out about this in IconStyles:
Code:
<IconStyle BackGlowRect="212,170,254,212" BackGlowResource="/images/icons/icon_ss9.dds"
FrameRect="126,42,168,84" FrameResource="/images/icons/icon_transp1.dds"
MouseOverRect="84,84,126,126" MouseOverResource="/images/icons/icon_transp1.dds"
Name="self_spell" PressedRect="168,42,210,84" PressedResource="/images/icons/icon_transp1.dds"
ProgressRect="84,42,126,84" ProgressResource="/images/icons/icon_transp1.dds" />
It seems that the progress refers to the recast timer. But if you take a look at that DDS file, you'll see that 84,42,126,84 is pointing to a white square, not a black/grey one. Hope it helps a bit... trying to find out more.