When you logon with your normal Domain logon in a workgroup setting you are using an XP feature called "cached credentials." With cached credentials there is no necessity for a Domain authentication.
The reason that My Network Places can enumerate Workgroups is that, assuming Netbios over TCP/IP is enabled, in the absence of a WINS or DNS server XP will use the Computer Browser service to enumerate available Workgroups.
Moreover, it is not at all unusual to see Domain settings that also have additional Workgroups, all of which can be enumerated along with any Domain connections.
A Workgroup has no authenticating authority. All that is required for a Domain workgroup to participate in a Workgroup off-site is for the user to recognize that the Network model used is that of the traditional Win2k security model, or in XP terms, Simple File Sharing is disabled.
The local account logon is available on a Domain computer as long as the Domain Administrators have not removed all local accounts. At the logon screen under Domain: is a scrollable window that if scrolled will show the option of the local logon account "This computer."