R help - Sep 2008 - diff() for panel data

Hello, everyone!
I'd like to find out how I can do first log differences in a panel?
(The Penn World Table data that's available in the PWT package)
The regular diff() function ignores the country/index/"panel unit",
with depressing results.

A second request, how can I best "filter" the data (e.g. generate a
data frame with the data for a single country or a single year)?

Thank you!
Gabriel

I was suggested that more details with help re: my question on first
differences in panel data...
The data set in question is PWT6.2:
> str(pwt6.2)
'data.frame':   10340 obs. of  27 variables:
 $ country: Factor w/ 188 levels "Afghanistan",..: 1 1 1 1 1 1 1 1 1 1
...
 $ isocode: Factor w/ 188 levels
"AFG","ALB","DZA",..: 1 1 1 1 1 1 1 1 1 1 ...
 $ year   : int  1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 ...
 $ pop    : num  8150 8284 8425 8573 8728 ...
 ...
 $ cgdp   : num  NA NA NA NA NA NA NA NA NA NA ...
 ...
 $ grgdpch: num  NA NA NA NA NA NA NA NA NA NA ...

The panel has countries as units and years for time.
What I want to do is have a fist different of logs of cgdp and pop.

The reason why I can't use diff(log(pop)) is because when the data for
a country ends, e.g. Afghanistan 2004, the next observation belongs to
another country, e.g. Albania 1950, and R will do a first log
difference between the population of Albania in 1950 and that of
Afghanistan in 2004, instead of NA (since I don't have data for
Albania 1949).

I need a diff() that's aware of the panel structure of the data (i.e.
the countries).
The plm package does this, but only in a regression. I found no
function that I can use to save the log differences.

Thank you for your time,
Gabriel