heterogeneity in random effects variance

Dear *all*,
(how many.. seeing that this appears to be the first message? :-))

My question concerns LMM and it is not strictly related to developments 
   of the lme4 package.

In a LMM framework is it possible to set a heterogeneity model for the 
random effects? More precisely I am interested in fitting a LMM where 
the random effects:

u~N(0,D(a,b))

where the covariance matrix D is diagonal and depends on two, say, 
parameters a and b to be estimated: D=diag(s1,s2,..,sj,..) and 
sj=exp(a+bxj) with some known values x1,x2,.xj

Is it possible with lmer() (in lme4 package) or even lme() (in nlme 
package)?

I hope to find an answer in this new promising mailing list

Many thanks in advance,

vito

====================================
Vito M.R. Muggeo
Dip.to Sc Statist e Matem `Vianelli'
Universit? di Palermo
viale delle Scienze, edificio 13
90128 Palermo - ITALY
tel: 091 6626240
fax: 091 485726/485612