select observations from longitudinal data

    times = 3:4
    do.call(rbind, by(data, data$id, function(data)
        with(data, {
            rows = (time == times[which(times %in% time)[1]])
            if (is.na(rows[1])) data.frame(id=id, time=NA, x=NA) else
data[rows,] })))

    #   id time  x
    # 1  1    3 23
    # 2  2    3 13
    # 3  3    3 15
    # 4  4    3 27

is this what you wanted?

There's also the straightforward answer:

sapply(split(data,data$id), function(d) { r <- d$x[d$time==3]

+    if(!length(r)) r <- d$x[d$time==4]
+    if(!length(r)) NA
+    r})
 1  2  3  4
23 13 15 27

or, just to checkout the case where time==3 is actually missing:

sapply(split(data[-c(6,13),],data$id[-c(6,13)]), function(d) {

+    r <- d$x[d$time==3]
+    if(!length(r)) r <- d$x[d$time==4]
+    if(!length(r)) r <- NA
+    r})
 1  2  3  4
23 14 15 NA