You have always been able to build anything on top of it without concerns about patents from the implementation. This claim was so hilarious that it is incomprehensible that anyone ever believed it.
You have also been able to reimplement .NET CLR and core libraries without fear of patent litigation (from Microsoft). They have placed the CLR and core libraries under the legal estoppel of the community promise since 2007 (IIRC), in addition to publicly granting patent license to anyone creating an implementation of NET CLR and core libraries from the specifications. This latter was part of the process under which C#, .NET CLR and core libraries was standardized under ISO. A precondition for the standardization was that any necessary patents (for implementation) be offered on RAND terms (reasonable and non-discriminatory). Microsoft has always offered the patent grants free.
The community promise was created in response to FUD from (among others) FSF that Microsoft would just sue anyway (despite patent grants), and with their vast army of lawyers and deep coffers they could bury in court. The community promise creates legal estoppel, whereby a case by Microsoft would be dismissed if you acted "in good faith" by relying on the promise.
Open sourcing Parallel LINQ has no bearing on the patent status of anything building on top of it. If you believe the FSF FUD, you will be ensnared in .NET technology and Microsoft will sue you out of existence if you ever become successful. If you do not believe the FUD, you can continue to take advantage of LINQ, Parallel LINQ, .NET, C#, F# etc.