Three possible issues (none of them mutually exclusive):
1) DNS/Name resolution:
Q: Can you ping the "server" by hostname?
2) Windows 7 IPv6:
IPv4 and IPv6 are both enabled in Win7; ipv6 is, by default, the *preferred* protocol. Here are a couple of examples of file sharing issues under Win7/ipv6; you might well be encountering something similar:
3) Windows 7 Security:
This is the "biggie". These links might help:
Console Root -> Local Computer Policy -> Computer Configuration -> Windows Settings -> -> Security Settings -> Local Policies ->Security Options
When you're there change the following policies
3) Microsoft network client: Send unencrypted password to third-party SMB server: Switch it to "Enabled".
4) Network security: LAN Manager authentication level: Select the option: Send LM & NTLM - use NTLMv2 session security if negotiated.
As you've found, the use of an IP address or a FQDN (host.domain.com) as the target causes a different security path to be taken compared to a single-label host. Adding domain membership introduces another layer of complexity because IP/FQDN access will then rely on Kerberos while single-label still falls back on NTLM.
These settings can mostly be controlled through policy, so joining a "real" domain with multiple layers of group policy settings may produce varying results, depending on what the domain admins have specified.
Please post back what you ultimately find resolved the problem!