[R-sig-ME] post hocs for lmer / lme

[Kay Cecil Cichini]

hello,

i searched for a reference how to do post hocs for lmms or glmms but 
without success!

i have a model with 2 nominal explanatory variables (2 levels and 4 
levels) and 2 nested random variables. from my final model i want to 
examine certain combinations of the fixed factors.
how is this best achived in LMMs / GLMMs?
is it valid to re-order the levels of the factors and assess the 
t-statistics of the paramteres?

any advise or pointers on useful references would be greatly appreciated!

best,
kay