Press ALT+B in 3d window, draw a rect, and it becomes a clipping
volume of 4 planes. You then can rotate the view anyway you like.
Works for each 3d window individually.
Disable it with another ALT+B press.
Commit is huge because it had to change all selection code as well.
The user-clipping planes are in 'eye space', the other clipping
happens in projected 'viewport space'.
Nice to notice is that the 'x=3200' convention (to denote a coordinate
is clipped) now is a define. Define value is still a number though... but
we now can get up to screens of 12000 pixels without issues!
Known issue; here it refuses to draw the 'object centers' or Lamp icons
within the clipping region. Can't find any reason for it... however, we
might move to non-pixmaps for it anyway.
Testing might reveil numerous issues, will be standby for it.
Curious? Check this http://www.blender.org/bf/rt4.jpg