I'm starting to use LINQ as a true query language in code to help improve readability. Until recently I was afraid to touch LINQ because of the LINQ to SQL team move under the Entity Framework team (trying to ignore that conversation here) -- will LINQ the query language be a safe bet going forward (as much as anything in this fast moving industry) ?
It's worth distinguishing between "LINQ" and "a particular LINQ provider". I think it's safe to say that LINQ itself is here to stay - and it's phenomenally useful for in-process collection processing via LINQ to Objects.
As for which LINQ provider will "win" (if any) - that's a harder bet to call.
I would certainly learn the fundamentals of LINQ itself though - and LINQ to XML is a lovely XML API as well.