Blue Mars only does collada imports. Both still and animation file imports must be collada based.

Second Life supports obj imports.

I don't believe Blue Mars has any plans to support obj format in the future. Blue Mars developer lists these eight 3rd party tools:
1. Maya
2. 3ds Max
3. Blender
4. Shade
5. SketchUp
6. Lightwave
7. Collada
8. AC3D
All of these programs have collada exports.(Collada.. funny, right?). On a side note, Shade doesn't seem to trust SketchUp or Blue Mars since it has individual exports for both.
The Blue Mars developer guidebook also gives directions for collada in Maya and 3ds Max and mentions nothing else.

MoI -> Blue Mars would be a lot of fun, though.