I did do a fair amount of googling prior to posting that question. However, I have come accross some answers. Rather than delete my post - I guess I should share the information. If others thought the information was clear, please delete this thread!
Firstly, there is a page identifying how to add custom IDP's: https://sites.google.com/site/gitooldocs/customidps
There is also a sample site (http://www.openidsamplestore.com/localmapping/) which uses facebook.
How does the advanced demo work for identity providers who are not
E-mail providers, such as social networks?
The hardest part about
designing the advanced site was to find a way to handle all the
edge-cases that can happen with these types of identity providers.
Google previously published a summary of best-practices for
account-linking that describes why these types of identity providers
are so much harder to support. However this demo provides a user
self-service mechanism for all the tricky cases to avoid the costs
that a website might otherwise occur if those users contact a customer
support representative.
Finally, a best practices run-down is available here:
https://sites.google.com/site/oauthgoog/UXFedLogin/loginlogic
EDIT 1 :
If that identity provider asserts email addresses that it does not
host, we suggest you also implement additional account linking logic.
A future version of GITKit will add support for these type of
identity providers, such as social networks, which will avoid the need
to implement that logic
Perhaps GITKit is the future after-all... Would be nice to have an idea of the time-frame in which this support will be added though...
EDIT 2 :
Direct from the horses mouth (Eric Sachs @ Google - Source Link):
That feature is not expected to be generally available in 2011. We
are shooting for Q1 2012