You have to use 'finishopacity' with 'opacity' in order to get even opacity across the picture.
By the way, transparency doesn't work all that great in IE 6 either. I use Bob Osola's JavaScript fix for this, works great!
I had this same issue -- all the IEs would fail, but Firefox and all other browsers would not have problems.
The way I fixed it was to open up the PNG in Gimp, choose the Fuzzy Select Tool, set the threshold to 150%, check Antialiasing, uncheck Feather Edges, check Select Transparent Areas. Next, I clicked on the transparent areas -- all the ones I could find on the image and clicked the Delete key (to mean "Clear"). Then, I resaved the image again. This resolves the problem about 98% for most images in all the Internet Exploders.
I want to caveat that instruction a little, though. If you choose Fuzzy Select and it ends up selecting more than the previous transparent area, then set to 3%, fuzzy select, click Delete, then reselect again with fuzzy select at 150%, then click Delete, and it should be resolved without deleting any of your image.
If you don't have Gimp, it's cross-platform and free for Windows, Mac, and Linux.