The invert icon is really complicated. Although I'm not sure inverting the colors or adding a line over the words would be easily understandable.
An ideal solution would be to change the text of conditions to the inverse sentence like "Is overlapping Object" by "Is not overlapping Object", or for comparison conditions change it to the opposite operation (x<y to x>=y). But from what I know about the internal structure of plugins I'm not sure this would be feasible. So what is left is convey it with the icon.
The "invert" operation in C2 are thighly related to concepts like inverse, opposite and negation. They are all very abstract and because of this are really hard to represent.
The previous icon for invert was the 'not equal' symbol. It was a bit unrelated to the previous concepts, and could be potentially conflicting with the comparison operators. So I tried to think in other alternatives.
Looking for existing conventions I remembered the logical NOT gate symbol, that is used for logic inverse operations. At first it seemed like a good alternative, but it's too distant from the repertory of common people. It's meaningless to most people, and is not even possible to deduct what it should mean just by looking.
I found more known representations within the negation concept. From this I found two alternatives: the prohibition symbol, and the cross that represents negation. The prohibition symbol seemed more fitting for disabling events, so I choose the negation cross for invert. It also happens to be used in the equivalent "invert" of Multimedia Fusion.
Within the options I found the current icon seemed the best pick.
<img src="http://dl.dropbox.com/u/7871870/Scirra%20graphics/icons/invert-icons-01.png" border="0" />
I thought in some other alternatives that used "black VS white", but they all conveyed more the concept of "contrast" than inverse. There's also Yin Yang symbol, but I don't know how much people grasp it's concept.
If you have other good suggestions I would be happy to know.