I've noticed that you don't have a section which covers this on the website or at least I haven't found it on there, I think this would be a good one to add. Anyway if people have used Dreamspark to get their copy of Server 2008 & R2 then they will notice that on a reinstall activation will not work and will ask them to call Microsoft, also if they click Activate via telephone the system will ask you to change your product key. There's a simple way around this by doing the following before activating.

Windows Key + Break to open System Properties.
Scroll down to Product Key and click Change product key.
Enter your product key.
When activation is shown click Telephone activation.
Write down or type into notepade the code given back to you on the phone and store it.

Next time you reinstall simply do the same as above but don't actually call the activation centre because unless you've changed hardware the code will be the same every single time just like it is on Vista & 7.

Didn't know that, but as the code is dependent of the hardware, this makes sense. Thanks for your explanation! I won't add it to the manual, because this isn't Windows Server specific, but people can find it in this topic.