I got one of these RETAIL 1.7v Celeron 566 just before Christmas. Matched to my AOpen AX6B and AOpen FC-PGA Converter Card it runs all day without any problems at 875mhz using the 103mhz bus speed. Tried 112mhz bus (the smallest jump I could make) and it failed to post, so I backed down to 103mhz.
NOTE: This was at the default 1.7v - haven't even attempted to bump the voltage up, nor will I as long as it's stable at 875.
One thing to note however. Even at this speed most games will only benchmark to about the level of a 600-700 P3, so that was a bummer. However, I ONLY PAID $71 FOR THE RETAIL CPU at buy.com.
Overall, very happy!