Two of my ISVs officially on the beta program asked me the same question end of last week "where is mobile?".
In my head I had "SQL Server 2005 Mobile Edition beta 1 is aligned with SQL Server 2005 beta 2". However I had not gone looking. So turns out it is an additional download:
Details and link to a webcast can be found here http://www.microsoft.com/sql/ce/mobility/beta1.asp