This is slightly off-topic but this is an interesting article recently published in Technet which is worth scanning and bookmarking for reference: Active Directory Maximum Limits Here are some of the key bits...
"..domain controllers are limited to creating approximately 2 billion objects (including objects that are created through replication). This limit applies to the aggregate of all objects from all partitions (domain NC, configuration, schema, and any application directory partitions) that are hosted on the domain controller."
Max No. of Domains
"For Windows Server 2003, the recommended maximum number of domains when the forest functional level is set to Windows Server 2003 (also known as forest functional level 2) is 1,200."
"...in a Windows Server 2003 domain, we recommend a limit of 1,200 domain controllers per domain to ensure reliable recovery of SYSVOL."
"There is a limit of approximately 1 billion security identifiers (SIDs) over the life of a domain."
FQDN Max Length
"Fully qualified domain names (FQDNs) in Active Directory cannot exceed 64 characters in total length, including hyphens and periods (.)."
OU Max Length
"The maximum length for the name of an organizational unit (OU) is 64 characters."
All part of 'Planning an Active Directory Deployment Project' @ http://technet2.microsoft.com/windowsserver/en/library/d2fc40d8-50ba-450c-959b-28fd7e31b9961033.mspx?mfr=true