A little googling reveals that Cheetah only imports a small subset from SVG, namely only <path/> elements (or spline elements). It ignores all other elements, including lines.
See also:
http://cheetah3d.de/forum/archive/index.php?t-214.html
"The problem is that it is important to convert a object (circle, rect, text, etc) to a path. Since Cheetah3D only reads the path informations from the .svg file."
http://cheetah3d.de/forum/archive/index.php?t-434.html
".svg files can contain rects or text. But Cheetah3D only imports the Bezier paths from the SVG file. So convert the rect in Illustrator to a path and Cheetah3D should be able to read it."
