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:
http://www.microsoft.com/downloads/details.aspx?FamilyID=0a6174a4-c009-4768-8284-698c32ec84e3&DisplayLang=en

Details and link to a webcast can be found here http://www.microsoft.com/sql/ce/mobility/beta1.asp