For best pratice, by default, let exchange servers to pickup DCs and GCs automatically, However, for some case, you may have to force Exchange 2007 to use a static Global Catalog
Set-ExchangeServer -Identity EX-HUB01
-StaticDomainControllers:dc1.domain.com,dc2.domain.com,dc3.domain.com,dc4.domain.com
-StaticGlobalCatalogs:dc1.domain.com,dc2.domain.com,dc3.domain.com,dc4.domain.com -StaticConfigDomainController:dc2.domain.com
-StaticDomainControllers:dc1.domain.com,dc2.domain.com,dc3.domain.com,dc4.domain.com
-StaticGlobalCatalogs:dc1.domain.com,dc2.domain.com,dc3.domain.com,dc4.domain.com -StaticConfigDomainController:dc2.domain.com
To Exclude DC's
Set-ExchangeServer -identity <server> -StaticExcludedDomainControllers:<domaincontroller>;<domaincontroller>
Get-ExchangeServer cmdlet afterwards, the DCs/GCs that I
listed in the Set cmdlet, are NOT listed.
You will need to add the -Status switch to the command to see the DCs.
You may also need to know
How to configure Outlook to a specific global catalog server or to the closest global catalog server
http://support.microsoft.com/kb/319206