Does mirror modifier not work on planes? [2.49a]

The mirror modifier doesn’t seem to work on planes in 2.49a.

Is that true or am I doing something wrong?

Works fine.

Just to be sure, you did actually edit the plane in some way so that the mirror doesn’t sit on exactly the same space as the original, right?

All I did was move it over to one side so the mirror would be on the other side.

That should be all I need right?:confused:

EDIT: I guess you do have to edit.

Mirror modifier works on the object’s pivot point.
Probably you moved the Plane in object mode and the pivot remained in the center of the object. That is why mirroring seems like not working.
Prior to mirroring, place the pivot point of the object where you whant the mirror plane to occur (use Center Cursor button). Or translate the object’s vertices in Edit mode.