I assume you're talking about IE6 not supporting alpha-channel transparency and IE7 getting the gamma wrong?

Both of those problems can be easily fixed (in most cases) by using iepngfix for IE6 and stripping gamma information from your PNGs (which you should do anyway, to minimize the size of your images). I've been using PNGs successfully in my projects for several years, and during most of those years I had to support IE6/7. In fact, if you need to support any version of IE, PNG should be the least of your problems.