Where can I find examples of code, written in "Unified Parallel C"?
I also interested in normative documents about this language (standards, reference manuals, online-accessible books and courses). What extensions were added to C to get UPC?
Is this dialect alive or dead?
UPC is still alive as a research project at UC Berkeley and is probably in use by high-performance computing facilities and national labs associated with the research group. You can get the latest version, released November 2009, here. Documentation, sample code, etc. can be found here. The language spec is here; there are many extensions on top of C99 syntax to support threads and inter-thread communication as first-class entities (perhaps not in the strictest sense of "first class", but certainly more so than in C, where you call out to opaque libraries to do synchronization and communication).
UPC is still in use some places; I am not an expert, but from what I can tell using UPC consists of a GCC-based compiler, a runtime, and a messaging layer GASNet that runs on top of your networking stack. These seem to be relatively well-supported on the types of machines for which you might want something like UPC. I wouldn't call it a huge developer community, but if you administer a large parallel machine, it's worth giving it a shot. If you just want something to work on your laptop, desktop, or server box, there is a wide variety of other parallel programming models with commercial support, tools, etc.