[R-sig-ME] How to get dispersion parameter from a binomial mixed model?

[v_coudrain at voila.fr]

I am modelling data with a generalized mixed model with binomial error distribution and I am concerned about overdispersion. I know that dispersion parameter can 
be measured as deviance/df, but for mixed models (= with random effect), the number of degrees of freedom cannot be extracted (I am using the function lmer from 
R). Is there any way to find out if my data are overdispersed?

Thank you very much
Valerie
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