Access denied when running Warmup script

Hello,

I am trying to run the warmup script:
http://kirkhofer.wordpress.com/2008/10/18/sharepoint-warm-up-script/

in our production platform using PowerShell for SP2010.
When I run it, I got the following error message:

Exception calling "DownloadString" with "1" argument(s): "The remote server ret
urned an error: (401) Unauthorized."
At C:\scripts\warmup_SP.ps1:26 char:26
+ return $wc.DownloadString <<<< ($url);
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : DotNetMethodException

This occurs for all site collections except the administrative site.
The same script is ran successfully in our dev. platform.
Can someone be a help please?

Thanks
Siav