[Illustrator CS4] Opacity Masks and Colour Inversion

[Flay]

I'm new to illustrator, so bear with me...

I'm dicking around with opacity masks at the moment. What I'm trying to do is fairly basic: create an object that, when placed over a piece of type, will invert the colour of that type. I want half of the type (the part not covered by the opacity mask) to be black on white, and the part covered by the mask to be white on black. From what I can gather from google, the 'invert mask' option should do this, but that doesn't seem to work.

Could someone please explain how I'd go about doing this?

[hewligan]

The invert mask for an opacity mask is just for determining whether the black bit or the white bit is fully transparent.

As far as I know, there's no way to create an object that inverts what's under it in Illustrator. You just need to go ahead and make half your text black on white, half of it white on black the non-fancy tricks way.

[Flay]

Dangit... I've been misinformed. Oh well, easy enough to do the normal way.