I've been running a Cel2 566/ZM6 setup (and have successfully had it running at 1014 Mhz, w/peltier rig) for quite some time.
The latest Bios upgrade will support up to a Cel2 600 (the older cB0 stepping series).
It does look like the newer cC0 stepping series (SL4NX) will NOT work with these BM6/ZM6 boards - I'm very sorry to say.
Technically, I've not seen any data as to why this is (hint to the "Tech" sites: Howz about an article on, Why?) but I've got two non-sequencial SL4NX 600's, and neither will fire up my ZM6 with SU bios.
If I'd known this problem existed (see, "hint to 'Tech' sites" above)I'd have gone ahead and switched to Duron. I mean, now I'm going to have to buy a new mobo anyway......sheesh.