Unfortunately, one limitation of declarative workflows in SharePoint is that you cannot associate these to content types.
One of our partners, OBS, have come up with a way of deploying a Nintex Workflow as a SharePoint Feature. When activated, the feature creates a list of a certain type and associates the Nintex Workflow programmatically in the feature activation event handler. This would allow you to easily deploy a Nintex Workflow across multiple sites.
Although this is not as convenient as associating to a content type, it does allow for a workflow to be enforced on a certain type of list and deployed via a feature.