With the ability to have separate workspaces, lead partitions, and landing page domain aliases, it really seems to be a major gap that you can not have multiple branding domains on the account to configure with your multiple workspaces. The limitation of one branding domain provides a less than ideal brand experience to the customer, especially if a brand like P&G, GE, etc were to want to use Marketo... I can't see one branding domain working for them.
It would be awesome if the Marketo team adds in the ability to configure multiple branding domains and select those as needed for your various marketing efforts to provide a seamless experience to the customer.