It IS a BIOS issue, but that can corrected with a simple flash update. The main problem is the voltage issue.
Check my sig. I have a Coppermine Celeron running right now at 880 MHz (8X multiplier at 110 MHz), on a P2B revision 1.10 with BIOS 1012 (updated from 1004).
All 1.12 P2B boards support Coppermine. Boards back to 1.02 also support Coppermine, but with some caveats, with voltage in the early boards being the most problematic. These will not always go under 1.8 V, and the newer ones do it kind of strangely, so Asus just states it doesn't work. However, if you update to the latest BIOS, essentially all of these work at 1.8 V and up for sure, and some will work at lower voltages too. My 1.10 works properly at lower voltages that are multiples of 0.1 (tested down to 1.5 V), but I run 1.8 V simply because I need it to hit 880 MHz. (I can do 800 MHz at 1.7 V though.)
Note that the slocket does not correct the problem. If your board does not support 1.7 V and you set the slocket to 1.7 V, it simply won't boot. The benefit of having the slocket is that it allows you to set the "wrong" higher voltage of 1.8.
What version of the P2B do you have?