My deferred engine developed on Geforce 8 is displaying weird stippling artifacts all over the screen on the Radeon 4850 with Cat 9.2 (Open GL 2.1 context). No issue with nVidia, but horrible grainy pattern on the Radeon. I have tracked this down to the full screen lighting phase of the deferred renderer where I enable stencil test and then copy a full screen quad.
I suspect that ATI are not properly supporting the DEPTH24_STENCIL8 as part of the Framebuffer.

Has anyone else used D24_S8 on the Radeon 4xxx ?

On a similar note, has anyone got a quick way to 'see' the contents of the stencil buffer. I.e. copy the stencil to a texture so I can visualise it's contents.