[sparky]

I'm not understanding what underlying datastore you're referring to in the case of LINQ-to-Objects or LINQ-to-XML. These frameworks perform "queries" on ordinary objects or XML data stored in memory within the .Net process, not on some other machine as with LINQ-to-SQL. You use LINQ-to-Objects in place of a filtering foreach loop, and LINQ-to-XML in place of XPath et al.

PLINQ is just a parallel implementation of LINQ-to-Objects and LINQ-to-XML. It works in the same scenarios .Net programs work in, i.e. multicores or SMPs running Windows (not sure if PLINQ works with Mono).