I currently manage a Windows Server 2003 domain with XP machines. I want to start migrating to Vista machines from XP. Can I utilize the GPO features of Windows Vista without having Windows Server Longhorn? Also can a Windows Server Longhorn replicate GPOs with a Windows Server 2003 domain controller?

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.

Running Longhorn BETA thte answer is yes to 2000, 2003 , xp and vista gpo's