The latest version I can see for ORM is 2008.3 1421 (Feb 23, 2009) - though I can see 2009 Q1 for everything else.
Actually I discovered that the manual inclusion of the line:
add assembly="System.Core, Version=188.8.131.52, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
(ie Asp.net 3.5 in my case) in the web.config, appears to solve the problem.
I prefer to use the full version as although initially the intention is to use it to access a mySql database (which is covered by the express version), the project also uses Sql server 2005 (full version) currently by means of LinqtoSql but eventually I want to migrate that also to use ORM.