Harold:
Could you try the same set of models using lmer2?
In July last year Sundar Dorai-Raj reported that the weights argument
was not being used in the CRAN version of lmer (lme4_0.99875-6).
Therefore, it's possible that you have actually observed the same
phenomenon in lme4_0.99875-9.
Sundar found that lmer2 did use weights, but it's not clear whether
the weighted model is correct. The development version of lmer
(lme4_0.999375-13) is much closer to the CRAN version of lmer2 than
lmer.
See his original post at:
https://stat.ethz.ch/pipermail/r-sig-mixed-models/2007q3/000262.html
Best wishes, Nick