Win2k3 and Win2k8 in same domain

We are planning on upgrading 2 of the 3 DC in our AD. We currently have 3 Win2k3 AD Domain controllers in our forest. Can I mix 2003 and 2008 DC’s in the same domain? I assume this is ok, but wanted to find out if there are any compatibility issues mixing servers. We plan on replacing two of the three servers with win2k8. The clients are all XP now but sometime in the next year we will be introducing Vista clients.