I have trawled Google for hours looking for this problem. I was trying to unpackage a .daa disk image created in PowerISO on Windows. I didn't want to pay for the PowerISO application for Windows (and my file was over 300MB, which is the limit for the trial), so I was hoping to find a Mac alternative. Pages and pages of Googlism later, and I found the open source (GPL) DAA Converter.

DAA Converter converts the archive to an ISO disk image format that you can mount in OS X. It's working great on Leopard, and I can't see a reason it shouldn't work on Panther or later, as stated on their page.