Useful post on the Microsoft Volume Licensing Blog covering dual access rights with Office 365. Three key takeaways:

1. There’s a good table showing which plans equate to which CALs (note that not all Office 365 plans include dual access rights)
2. Underlying on-premise servers need to be licensed for users (SQL and Windows Server to support SharePoint, for example) and
3. Office 365 USLs can be used to access licensed servers deployed on third party servers (licensed via Licence Mobility). http://bit.ly/1cdivtH