According to what's supposedly a Sony Store employee blog, no more PSPgo shipments are planned and the portable is apparently out of production. Over on the official Japanese Sony Style page, it is not in stock and shipments are TBA.
When asked about this, Sony replied, "It is a very exciting time for PlayStation portable devices. Before the end of the year we are launching NGP, our next generation portable device, which we believe will revolutionise portable gaming. In the meantime, the current generation of PSPs continue to be in demand, especially since the introduction of our value for money, Essentials range of games and we will continue to meet that demand."
The PSPgo was revealed at the 2009 E3 gaming expo in Los Angeles and went on sale later that year.