Standard PNG (as far as I know) does not support layers. Macromedia's Fireworks uses a proprietary version of PNG as its native format and it does support layers. I believe Photoshop can not open Fireworks PNG's. Can your client supply the images as PSD's?
Fireworks PNGs can be opened by about any graphic app. *but* the image will be displayed as a single layer 32 bit file in the current saved state.
Of course, your client could also do an "export layers as files" command from Fireworks.
Fireworks is a marvelously flexible and powerfull graph. app. for multimedia work, but strangely often ignored... (does it remind you something in the NLE field? :-) )
I haven't done this but it looks like either Photoshop or ImageReady can export animations frames as files. From the Photoshop Help:
"In Photoshop, you can export animation frames as files in PSD, BMP, JPEG, PDF, Targa, and TIFF formats. In ImageReady, you can export animation frames as files in GIF, JPEG, PNG, WBMP, and SWF formats."
Photoshop CS2 can directly export each layer as a separate file and maintain the layer transparency. Did this last week with a JuiceDrop for use in Vegas 7b. Required 47 video tracks in Vegas to build up the composition - no problems encountered.
What i like the most with Fireworks: vector/bitmap integration and the possibility to organise all the work with objects, layers *and* images. You can assemble a multi-object/multi-layers document organized in multiple "images" (or "pages") and export everything in one mouse click.