You can certainly use the GPO's of Vista from a Server 2003 domain, though you'll probably want to manage them from a Vista client to access all the settings. There is loads of advice on TechNet here, I'd strongly advise setting up a central store for ADMX files first, as described in the linked article.
I presume Longhorn will be capable of running in a mixed 2003 environment (as 2003 was with 2000) with the option to enable more features once all DCs are migrated to Longhorn.
