IN PROC GLIMMIX (SAS/STAT13.2 and later, I think), there is an option KR2 for degrees of freedom that is substantially more robust to the assumption of normality, based on a correction for nonlinear covariance structures. It does still assume that the distribution being fit is from the exponential family, so if you roll your own link or variance function, be careful about applying this method for calculating degrees of freedom.
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