Linked clone pools provisioned by Composer require a domain to be specified as part of the customization. The older full clone pools only require a vCenter Customization Specification to be used however, and it's not required to specify a domain as part of that. You can also set up your VMs by hand or an external tool and add them to a manual vCenter pool instead of provisioning them through View.
Note that if they are not joined to a trusting domain your users will need to authenticate twice when connecting - once to the connection server and a second time to the desktop VM itself with local credentials.
Mike