This discussion about activating the Volume License (VL) version of WinXP is a little confusing - the VL version of XP does not need to be activated. The installer gets the key, and that's it - there's no communication with Microsoft. If the key is valid, the installation is 'activated'.
Microsoft does however blacklist VL keys that it finds have gotten out into the wild. That blacklisting can occur in a service pack update or via the infamous "Genuine Advantage" check.
There may be some confusion regarding non-VL WinXP keys that you get from MSDN - those keys allow you activate something like 10 times, so they may seem like they're VL keys because they allow for multiple activations.
Note that the volume licensing mechanism for Vista is completely different and requires that a site have infrastructure (ie., a validation service) to support the licensing scheme.