Hi Evan,
thanks for your reply. It is a pity to see that Office 365 does not allow (yet, I hope) this login methods, as the 2 other major players are already doing.
As an alternative, I found some sort of workaround using a Windows Azure feature:
msdn.microsoft.com/.../dn151790.aspx
What do you think about it? Do you think it's feasible as a solution?
Thanks for your feedback,
Christian P.