Basically they use what's known as a corporate edition of Windows. It's a special volume license key that allows you to bypass the activation.
Some OEMs will modify the installer program to run a special program that does the activation if it detects you're running a system from that OEM. Dell does that, since I once accidentally used a laptop OS CD to install XP onto a desktop system that wasn't Dell. Didn't really notice until it kept rejecting my activation attempts, which made me look a bit more closely.
But you're not going to be able to get permission to do this unless you're a multi-million dollar client. If you're going to roll out a bunch of systems with identical configurations, consider investing in a copy of Norton Ghost or a similar program. If you have a valid license for each system, there's nothing wrong with you creating an image of one system, and then using that to roll out say a lab full of systems in a matter of hours instead of days.
When I build my own PCs and install the OS, early in the process, I must type in the product ID code. I occasionally configure new OEM builds for a local school. These come with the OS fully installed, drivers loaded, etc. But, I still have to install the product ID code on the first boot. Of course with XP, I must activate afterwards. Does anyone know how OEMs do a complete OS installation without the product ID?