How can I extract the AIC score from a mixed model object produced using lmer?

I am running a series of candidate mixed models using lmer (package lme4)
and I'd like to be able to compile a list of the AIC scores for those
models so that I can quickly summarize and rank the models by AIC. When I
do logistic regression, I can easily generate this kind of list by creating
the model objects using glm, and doing:

md <- c("md1.lr", "md2.lr", "md3.lr")
aic <- c(md1.lr$aic, md2.lr$aic, md3.lr$aic)
aic2 <- cbind(md, aic)

but when I try to extract the AIC score from the model object produced by
lmer I get:

md1.lme$aic

NULL
Warning message:
In md1.lme$aic : $ operator not defined for this S4 class, returning NULL

So... How do I query the AIC value out of a mixed model object created by
lmer?

<<->><<->><<->><<->><<->><<->><<->>
Peter Singleton
USFS Pacific Northwest Research Station
1133 N. Western Ave.
Wenatchee WA 98801
Phone: (509)664-1732
Fax: (509)665-8362
E-mail: psingleton at fs.fed.us