The ARB_vertex_program spec states that user clip planes should work with vertex programs provided that ARB_position_invariant is used.
This has worked fine for me since Geforce FX series, but the clip planes do not appear to be active at all with the new G80 cards no matter the driver version.
Is this a bug or has something changed that I need to fix?
Sounds like a bug if a program behaves
differently on g80 than on previous hardware.
Will check, but if you have a repro case,
that would be appreciated. 
I observe the same problem (I posted a similar question three weeks ago but then I found this old thread). In fact it seems that the clip planes are active but change with changes of the ModelView matrix rather then staying fixed once set up.
As it seems to be a driver/hardware bug is there any solution to this problem in upcoming drivers (it is very urgent for me, I need to make a workaround for our customers if this is staying like this longer and 9 month seems a long time already since this thread has been started)?
BTW the problem exists also with Quadro 4600 boards (not really a surprise but people pay a lot of money for their drivers to be reasobly high quality).
dreld