How can one do an MCMC hypothesis test on a mixed effect regression model with random slopes? The library languageR provides a method (pvals.fnc) to do MCMC significance testing of the fixed effects in a mixed effect regression model fit using lmer.  However, pvals.fnc gives an error when the lmer model includes random slopes.  
Is there a way to do an MCMC hypothesis test of such models?
If so, how?  (To be accepted an answer should have a worked example in R)
If not, is there a conceptual/computation reason why there is no way?
This question might be related to this one but I didn't understand the content there well enough to be certain.
Edit 1: A proof of concept showing that pvals.fnc() still does 'something' with lme4 models, but that it doesn't do anything with random slope models.
library(lme4)
library(languageR)
#the example from pvals.fnc
data(primingHeid) 
# remove extreme outliers
primingHeid = primingHeid[primingHeid$RT < 7.1,]
# fit mixed-effects model
primingHeid.lmer = lmer(RT ~ RTtoPrime * ResponseToPrime + Condition + (1|Subject) + (1|Word), data = primingHeid)
mcmc = pvals.fnc(primingHeid.lmer, nsim=10000, withMCMC=TRUE)
#Subjects are in both conditions...
table(primingHeid$Subject,primingHeid$Condition)
#So I can fit a model that has a random slope of condition by participant
primingHeid.lmer.rs = lmer(RT ~ RTtoPrime * ResponseToPrime + Condition + (1+Condition|Subject) + (1|Word), data = primingHeid)
#However pvals.fnc fails here...
mcmc.rs = pvals.fnc(primingHeid.lmer.rs)

It says: Error in pvals.fnc(primingHeid.lmer.rs) : 
  MCMC sampling is not yet implemented in lme4_0.999375
  for models with random correlation parameters
Additional question:  Is pvals.fnc performing as expected for random intercept model?  Should the outputs be trusted?
 A: Here's (at least most of) a solution with MCMCglmm.
First fit the equivalent intercept-variance-only model with MCMCglmm:
library(MCMCglmm)
primingHeid.MCMCglmm = MCMCglmm(fixed=RT ~ RTtoPrime * ResponseToPrime + Condition, 
                                random=~Subject+Word, data = primingHeid)

Comparing fits between MCMCglmm and lmer, first retrieving my hacked version of arm::coefplot:
source(url("http://www.math.mcmaster.ca/bolker/R/misc/coefplot_new.R"))
  ## combine estimates of fixed effects and variance components
pp  <- as.mcmc(with(primingHeid.MCMCglmm, cbind(Sol, VCV)))
  ## extract coefficient table
cc1 <- coeftab(primingHeid.MCMCglmm,ptype=c("fixef", "vcov"))
  ## strip fixed/vcov indicators to make names match with lmer output
rownames(cc1) <- gsub("(Sol|VCV).", "", rownames(cc1))
  ## fixed effects -- v. similar
coefplot(list(cc1[1:5,], primingHeid.lmer))
  ## variance components -- quite different.  Worth further exploration?
coefplot(list(cc1[6:8,], coeftab(primingHeid.lmer, ptype="vcov")),
         xlim=c(0,0.16), cex.pts=1.5)

Now try it with random slopes:
primingHeid.rs.MCMCglmm = MCMCglmm(fixed=RT ~ RTtoPrime * ResponseToPrime + Condition,
                                   random=~Subject+Subject:Condition+Word, 
                                   data = primingHeid)        
summary(primingHeid.rs.MCMCglmm)

This does give some sort of "MCMC p-values" ... you'll have to explore for yourself and see whether the whole thing makes sense ...
A: It looks like your error message isn't about varying slopes, it is about correlated random effects. You can fit the uncorrelated as well; that is, a mixed-effects model with independent random effects:  
Linear mixed model fit by REML
Formula: Reaction ~ Days + (1 | Subject) + (0 + Days | Subject)
Data: sleepstudy

from http://www.stat.wisc.edu/~bates/IMPS2008/lme4D.pdf
