Somewhere a while back I thought I saw something that said you would lose a site's PR if you moved it to a different server. Is this true and if so would it come back after the next update?
I agree with TheHoff, though if your site is unavailable for a long period of time you may suffer SERP slippage which is far worse than any potential PR loss.
When you move a site, just leave the old hosting live until you see the traffic is all going to the new host, meaning the new DNS has propagated. Once you see no activity on the old site for a while, you zap it. Never lost PR on a hosting switch.