When creating a Rackspace VM, the user_data is passed to the API but it additionally needs the cloud-init disk enabled in a second option. (EC2 and others use an HTTP-based metadata service, but Rackspace uses a virtual disk.)

http://docs.rackspace.com/servers/api/v2/cs-devguide/content/Create_Server_with_configdrive.html shows both user_data and config_drive being enabled.