numbering observations: help please!

Dear Friends,

I have the very simple problem of needing to number observations in a data
frame. After scratching the rest of my hair off my head without inspiration,
I'm using a silly loop. I'm sure that there is a much more elegant and
faster solution - can anyone help?

Here is an example:

my.data <- data.frame (person=c(1,1,1,1,2,2,3,3,3,3,3,3,3,4,4,4))   # now I
want to number those observations sequentially
# for each person

my.data$item.number <- 0
for (i in 1:length(unique(my.data$person))) {
	my.data$item.number [which (my.data$person == unique(my.data$person)[i])]
<- seq (1:dim(tmp)[1]) }