I suspect the problem when speedstep is enabled could be for example when speedstep drops the system down to a 6x multiplier it also drops the vcore (you can find out the vcore it's dropping to by using CPU-Z).
When the system is overclocked for example 333 FSB instead of 200 FSB that 6x speedstep multiplier will result in a clock speed of ~2GHz instead of 1.2GHz and the 6x speedstep vcore which runs 1.2GHz reliably might not be enough to run ~2GHz reliably.
The potential for a problem to occur definately exists but I guess it's not a concern for those who run with speedstep enabled.