[R] problem with graphic devices

[Kuan(謝冠生)]

Version:
OS: Windows XP/CentOS 5.5
R: 2.11.1
Acrobate reader: 9.3.2

Question:
After using graphic devices, such as pdf(), png().... in R, nothing happened.

Screen Log:
--------------------------
> pdf("foo.pdf")
> plot(1:10)
> dev.off()
null device 
          1
>
>
--------------------------

There should automatically pop-up a screen and shows the chart, but nothing happened by using above code on my machine.
Do I misunderstanding anything???
I have tried the code under WINDOWS and LINUX and got the same result.

Kuan