In order to request a server/SSL certificate for a domain you first have to register this domain. An email will be sent to a privileged address (postmaster, webmaster... @mydomain.net). Then you can submit a CSR (c.f. SubmitCsr).
Before you delete a domain, make sure, you revoked all certificates for this domain. Otherwise your access to these certificates is lost. Even if you reregister the same domain, this is a new one from CAcert's point of view, but if you want to generate a new certificate, CAcert will complain, that an old one exists. You can't revoke these certificates, because they belong to the old domain, which you deleted. Ouch!
(so one has to wait for the certificate to roll out? - nagy.gergely)