If you read the Office 365 Deployment Guide, it will state that to support multiple top level domains you need a separate instance of ADFS for each domain. This can get very expensive in terms of hardware, as you would need to make each instance highly available. Typically this would mean four servers per instance, two ADFS servers and two ADFS Proxies servers in the DMZ.
This is no longer true as of ADFS 2.0 RU1.
The following link below outlines the steps required to setup your domains for supporting multiple domains. http://onlinehelp.microsoft.com/en-us/office365-enterprises/ff652560.aspx#BKMK_CreateOrConvertADomain
You can download the ADFS 2.0 RU1 update from the following location http://support.microsoft.com/kb/2607496.
This greatly improves the functionality of ADFS and certainly will minimise costs for many organisations.