I've written a few documents covering this, but never for the community, always an employer!

Well Clifton Hughes over at MS has taken opportunity to bring together all the nuggets of information on aiding ConfigMgr to support workgroup based or clients in untrusted domains.

This document covers the processes and considerations for managing clients in another un-trusted domain, as if they were in a workgroup, and/or to manage actual Workgroup Clients. In this documentation, Workgroup Clients is the term that is used, however, be aware that through these same processes and procedures, clients in an un-trusted domain can be managed in the same way, and with the same limitations as actual Workgroup Clients. Note below, that if you do not have one already installed in your environment, you will need the Server Locator Point (SLP) role if you decide to pursue this process.

This method requires a little more effort and planning that with a domain joined clients, because of the lack of discoverability, name resolution, and lack of support for Client Push installation method, however, it is doable  

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Read the complete post at http://wmug.co.uk/blogs/r0b/archive/2011/05/03/configmgr-client-in-workgroup-or-untrusted-domains.aspx