Consolidating Domains

Hi there. My company currently has 6 separate 2003 domains all connected via trusts, at locations across the UK. I am trying to consolidate them all into one domain (onedomain.local). My question is this: After consolidation, say site 1’s DC is down, will the client pc’s be able to automatically connect to site 2 and use their DNS/ DHCP info to gain fileserver access? Do we all need to be on the same ip subnet for that to happen? Many thanks in advance for your help