Are models on sketchup's 3D warehouse free to use for personal or commercial work? I haven't been able to find copyright information on the site.
I'm not a lawyer, but this is my interpretation:
The Services allow you to submit content, including 3D models in the SketchUp and Keyhole Markup Language (KML) formats. You retain ownership of any intellectual property rights that you hold in that content. In short, what belongs to you stays yours.
Rights granted to other end users of the Services. You give other end users of the Services a perpetual, sublicensable, irrevocable, worldwide, royalty-free, and non-exclusive license to reproduce, adapt, modify, translate, publish, publicly perform, publicly display and distribute Existing Geolocated Models, Existing Non-Geolocated Models, New Models and related content and derivative works thereof which you submit, post or display on or through, the Services.
You fall into the "other end users of the Services" group. So you have the right to pretty much do whatever you want. But you will not own the intellectual property rights.