8bit PNG images support two kinds of transparency - index (which is only 100%) and alpha. Index transparency is what most image editors export and I guess you are using that and your transparency is exported as entirely opaque pixels. You can't do semi-transparency with index transparency (unless you use filters or opacity but they are not part of the image).
Of the big image editors only Adobe Fireworks supports 8bit+Alpha format export, though you can convert 24bit PNGs to 8bit+Alpha PNGs with several freely available tools. 8bit+Alpha is supported by all browsers except IE6 but contrary to the 24bit PNGs which are rendered entirely opaque where the alpha resides, the alpha channel in 8bit+Alpha PNGs is rendered entirely transparent in IE6 which makes it better alternative when you want to support IE6 at least partially.
the Telerik team