In browsing through the SQL Server 2008 SMO Object Library I noticed there are a couple of additional DLL's. For example, the SQL Server WMI Management objects, formerly in Microsoft.SqlServer.Smo.dll, are now in Microsoft.SqlServer.SqlWmiManagement.dll. There's also a new one called Microsoft.SqlServer.SmoExtended.dll. I took a look in there and found this list of objects, which have been removed from Microsoft.SqlServer.Smo.dll:
This means that if you are doing backup or data transfer operations through SMO you'll have to be sure to add references in your projects to the SmoExtended dll in SQL Server 2008.
I expect there will be more changes as updated CTP's are released.