I assume you want to add a 2k8 DC to your 2k3 AD environment. This is totally fine. First you have to prep the domain and possibly the forest to enable the 2k8 machine to become a DC. Then, you promote the 2k8 machine to be a DC and everything works great.
You will not be able to upgrade the forest or domain functional levels to 2k8 unless you decomission your 2k3 domain controllers though.
In terms of group policy and other AD features - the GPOs will be supported by both DCs. Usually it is the targets (the managed servers) that will vary in terms of which GPOs are supported.
Example: A 2k8 DC running a 2k8 functional level domain will be able to apply GP to a bunch of 2k3 servers, but the 2k3 servers won't support the hundreds of new GP functions available in 2k8.
Example: A 2k3 DC will apply 2k3-functional level GP to a group of 2k8 member servers, but none of the new GP functions available in 2k8 will be available to deploy to the group (since the DC is 2k3).