Many Collada exporters have inverted opacities in the materials. This means that something that is 100% opaque will import at 0% and will, therefore, be invisible. Adobe has a fix for this with a script that inverts all of the Opacities in the materials. You can read about how the fix works and how to install it on the Adobe website at the link below. Please note that although the Adobe page refers to CS4, the fix also applies to CS5.

Adobe Tech Note for Collada 3D Opacity issue

Once you understand this script works and where to install it, download the script itself using the link below.  This script is also compatible with CS5.

Adobe Photoshop CS4 3D Invert Opacity script