I've downloaded QGIS-1.8.0-2.dmg and the required GDAL Complete 1.9 framework package and GSL framework. The frameworks are installed; however, when I try to start the installation of QGIS-1.8.0-2.dmg, I receive this warning: "...disk images couldn't be opened", reason: not recognized.
It sounds like you have a corrupted download of QGIS-1.8.0-2.dmg. The Mac OS X "disk images couldn't be opened" error is almost always due to an invalid .dmg file.
Try re-downloading it from kyngchaos.com's qgis page. If you didn't get the supporting frameworks and QGIS-1.8.0-2 installer directly from kyngchaos.com or another trusted source listed at QGIS.org's download page... I suggest you not install whatever you downloaded.
I had no problems doing much the same install but I installed the QGIS package BEFORE the GDAL and Frameworks packages. You might find that changing the order of installation might help.