PLINQ was added in the .NET 4.0 Framework as an extension to LINQ.
This is Parallel LINQ. It's a way to run LINQ queries in parallel on multi-core/multi-processor systems, in order to (hopefully) speed them up.
There is a good article on this in MSDN Magazine.
For current details and plans, I recommend reading articles on the Parallel Programming with .NET Team Blog. They are the team implementing the parallel extensions, including PLINQ.