IE 6 does have some problems with PNG's.
In the short term there's not an awful lot you can do about it. There is some code in NetTalk that can help (in some cases) but it doesn't work if the image is "hidden" as the page opens.
One option is to not use PNG, but use GIF - but of course GIF has only "hard" transparencies, and has a limited palette.
On the up side, IE 7 is an "automatic upgrade" for IE 6, so IE 6 folks should be encouraged to use it. Apart from anything else it's more secure.