The following code returns the time when a web resource was last modified:

static DateTime GetModificationTime(string url)
{
   WebRequest request = WebRequest.Create(url);
   HttpWebResponse response = (HttpWebResponse) request.GetResponse();
   DateTime lastModified = response.LastModified;
   response.Close();
   return lastModified;
}

It’s quite a simple piece of code actually and very similar to a sample in MSDN. But maybe it will be useful to someone since a member of my developer team was convinced that there is just no way to get this information in .NET.