library(bbmle)
f <- function() {
  maxit <- 1000
  d <- data.frame(x=0:10,
                  y=c(26, 17, 13, 12, 20, 5, 9, 8, 5, 4, 8))
  mle2(y~dpois(lambda=exp(lymax)/(1+x/exp(lhalf))),
   start=list(lymax=0,lhalf=0),
   data=d,
   control=list(maxit=maxit),
   parameters=list(lymax~1,lhalf~1))
}

f2 <- function(method="BFGS") {
    d <- data.frame(x=0:10,
                    y=c(26, 17, 13, 12, 20, 5, 9, 8, 5, 4, 8))
    mle2(y~dpois(lambda=exp(lymax)/(1+x/exp(lhalf))),
         start=list(lymax=0,lhalf=0),
         data=d,
         method=method,
         parameters=list(lymax~1,lhalf~1))
}

m1 <- f()
p <- profile(m1)
## FIXME: check results (need to save in an environment-friendly way!)
print(head(as.data.frame(p)),digits=3)

m2 <- f2()
p2 <- profile(m2)
print(head(as.data.frame(p2)),digits=3)