Revised this question to be more concise, consolidating several revisions.

Symptoms:

From a domain-member Windows 7 Client:

• Domain credentials to a domain controller => success
• Domain credentials to a member server (by hostname or FQDN) => success
• Domain credentials to a member server (by IP) => fail
• Local credentials to a member server (by either) => success

From a non-domain-member Windows 7 Client:

• Domain credentials to a domain controller => success
• Domain credentials to a member server => fail
• Local credentials to a member server => success
• (Identical behavior from a Mac RDC 2.1 client)

Server Configuration Details:

• Windows 2008 R2 Datacenter w/ SP1
• The domain in question is a subdomain of a Windows 2008 domain (forest root).
• Root has DCs in both Site A and Site B, subdomain only has DCs in Site B.
• RDP is operating normally on all root member-servers and DCs.
• No remote desktop settings are defined by GPOs.
• Network level authentication is enabled; all clients are compatible and the certificate exchange/SSL handshake completes successfully.
• Not catching any errors in netlogon log.