Just to correct a stat used. IE exhibits at least a 70% marketshare not 50%.
Transparent PNG's work in IE7 so I'll correct your stat and stand by my original of > 50% can use Trasparent PNG's ok now that IE7 has about 50% of the IE share by the sites I am involved in, and by transparent I mean 32bit and not the 8 bit that IE6 and below (except the Mac versions which handled it also) could handle without resorting the the MS CSS hacks.
To the original poster though, you may have to do some trickery like having 2 sets of images. one for your original and another for your .8 transparency. now fully understanding what you wanted to do I also believe you are going to run into a brick wall doing it the way you were trying.
The thing about hacks are they are hacks and they do not account for everything the user will try as you have found out.