[CentOS] Problem with Yum on CentOS 5.4

Hi,

I recently downloaded and burned the 5.4 DVD ISO. All my machines are 
now running 5.4, but these are installs I performed with 5.0, 5.1, 5.2 
etc. and then progressively upgraded. Now I had to use 5.4 because a 
friend's laptop, a brandnew Fujitsu Esprimo Mobile, refuses to boot 
anything under 5.4.

After a fresh install of the base system, first thing I do is 'yum 
update' with the default setup... only this time, nothing happens. I 
stays about a minute or two on 'Determining fastest mirror'... and then 
tells me :

"Could not retrieve mirrorlist..."

On the other hand, 'yum update' on all the other installs (from older 
CentOS disks) works correctly.

What's wrong?

Niki