LINQ is not just an ORM tool or library. I saw some LINQ providers like
LINQ to SQL (to connect to SQL Server Relational Database), LINQ to NHibernate (Hibernate's cousin),
LINQ to LDAP and so forth. In general, LINQ is a standard way to access your data without care about
where they are.

For adding LINQ to .NET Framework, Microsoft introduced lots of new features in C# language and almost
none of these new features is available in Java till now. So wait for Java 7, JEE 6 and JPA 2.