If you want to be able to see your bitmap reference in a perspective
view, do the following:
1: Place a bitmap (CMD: backgroundbitmap) in the orthographic viewport
of your choice (ie. front, top, etc.)
2: Right click the viewport name in the top left> go to 'Set View'>
change view to 'Perspective'
3: Your bitmap should now show up in the perspective view.
Unfortunately, it would seem that you can only place one bitmap
reference in each viewport.
Even Better:
Try the pictureframe command. Then you can use multiple views
simultaneously in any viewport. See Example:
http://www.tjpgraphics.com/assets/images/MB5_background_demo.jpg
-James