An interesting one this. I've recently set up a second 2008 based forest (say 2008.local) alongside the current 2003 forest (2003.local), both running at 2003 functionality level as they have a two way trust between them.
DNS wise, I've zone transferred one into the other and visa versa and everything appears cool in the fact that I can ping addresses in the other forest, both client machines and servers.
DNS is on both servers, DHCP is on 2003.local and both machines have static IPs.

The problem is that after a few days I can no longer ping the domain name 2003.local from 2008.local but I can visa versa. I can also still ping client machines from and to both forests fine.

The error that I'm greeted with is the standard 'Ping request could not find host 2003.local. Please check the name and try again'. The only way to temporarily fix this at the moment is to restart the 2008.local domain controller every so often!

What do we reckon?