What considerations should be made for a web app to be released on a cloud hosted system?

I have a web app that is primarily a WordPress app, but it pulls content from a Django app, simply by calling a service that uses Django models.

My understanding of cloud computing is a bit vague. If the site needs to scale up with short notice, does the cloud provider (Amazon, Rackspace, whomever) simply spin up new instances (copies) of my initially configured server? How is state managed between all of them? Are there any good primers on this subject? It's hard to find much out there without getting caught up in the marketing.

If you're not entirely sure about running servers, take a look at something like RackspaceCloud's Cloud Site product which will do all the scaling for you.

Edit: I've just noticed you're using Django and the CloudSites product doesn't support Python, so maybe that's not for you.