what are the system requirements for mac os x 10.6 server?

Click here for information. Mac OS X Server isn't intended for use on a portable Mac.

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While not 'intended', it is indeed possible on a portable. Exigent circumstances required me to re-role an aging MacBook Pro 2.16 Intel Core Duo to act as a server on a temporary basis.

It boots just fine from an external drive running Server 10.6.8. I also believe that installation (while not recommended) is not actually blocked.

Tip for the top: just be sure to pull the battery out of the laptop (assuming that's possible) to avoid turning your battery into a grossly distended, non-functional brick... they were not designed to be left on charge permanently!