I tried to search to see if there was a 3D file viewer request already here but the search tool only works with words 3 characters or longer, so 3D doesn't garner any results.
I would really like to see an app that I can view my 3D files. The way I imagine it would work is you pinch to zoom, 2 fingers to pan and one finger to rotate in any direction. I found an app called gIMD2Test that show a 3D model that can be viewed so I think it may be possible.
If you are still interested in a 3D viewer, there is
a viewer in beta testing. It allows the user to
download STL files and view them by zooming, rotating
and panning. It also provides other useful information
about the model. Almost all CAD and modeling programs
can generate STL files. It should be released in about
2 to 3 weeks.
The program will not be open source. It will be a pay app and price has not be determined. However, it will probably be about 3 or 4 dollars.
It is a derivative of a much larger PC based application for viewing full assemblies with animations for maintenance. The name has not been decided on but will have "STL" in the name. It can also display the 3D models on top of a photo or background.
The STL viewer for the iPhone was released in the App Store last week.
You can generate STL files from 3DS and then download them via a web site
to the iPhone for viewing.