glutBitmapCharacter and ClipPlane

I’ve a rather large project in which billboarded quads are tagged with a text label. The font bitmaps are build using glutBitmapCharacter, placed into 96 display lists and rendered with glCallLists (the straightforward method). Prior to rendering the label, I call glRasterPos3d to place the first character of the label near the left side of my billboard quad.

However, it seems that these bitmaps are not clipped by my only clip-plane, which is placed such that distant lables are clipped. I know the clip-plane is working, since it clips all my quads and lines correctly.

It worked on GeForce2, but doesn’t work on Radeon9700. Any ideas?