Of course it is not with parallelism in general. PostgreSQL is pretty good at the process model although this means that queries can't have portions executed in parallel (within the query). Multiple sessions with multiple queries can be executed in parallel with no problem however.

Note that the big open source projects (Gridsql and Postgres-XC) which get rid of this limitation currently do this through a different method without breaking single threaded process model.

So the question is how intimately connected portions that run in parallel should be.