lme RE variance computation

As I understand it lme (in R v1.9.x) estimates random effect variances 
on a log scale, constraining them to be positive. Whilst this seems 
sensible, it does lead to apparently biased estimates if the variance is 
actually  zero - which makes our simulation results look strange. Whilst 
we need to think a bit deeper about it - I still haven't got my head 
around what a negative variance could mean - does anyone know a 
way to take away the contraint and allowing zero or negative 
variances?

Steve.  Dr Steve Roberts 
  steve.roberts at man.ac.uk

Senior Lecturer in Medical Statistics,
CMMCH NHS Trust and University of Manchester Biostatistics Group,
0161 275 5192 / 0161 276 5785