I just bought a new ARM based HP 12c+ on Amazon but it has the old firmware, dated 2008-06-28.

What is strange is that this unit bears serial number 4CY14501C8 which, if I am not mistaken, indicates that it is Chinese built on the 45th week of 2011 (i.e. manufactured around Oct-Nov 2011).

I had been waiting to purchase a 12c+ to make sure it did not have issues with the old firmware. How is it possible that a recent unit would still be loaded with such an old version of the firmware?

The 12C firmware dated 2008-06-28 is the original version loaded on the 12C+ upon its initial release as shown in Katie's review when it was first released. Several subsequent firmware version seem to have been developed, as chronicled
here. There were reports (see
here and here, for example) that a version dated 2009-07-02 made it into the production chain. The latest reported version is dated 2009-11-19, but it appears that this version never made it into any production units.

After finding all of the above, it occurs to me that it does not address your question about how such a recent unit could be loaded with such an old firmware version. I also seem to recall discussion regarding units produced in different factories. Your unit was made in the 4CY factory, while others were made in the CNA factory. Perhaps the CNA factory got an update at one point to the 2009-07-02 firmware, but the 4CY never got anything beyond the original 2008-06-28 version. The recently released 30th Anniversary Edition had the 2009-07-02 firmware according to Katie’s post in this thread. The 30th Anniversary Editions seem to have been made in the CNA factory, according to
this thread, which also discusses potential quality differences between the factories.

Edited: 29 June 2012, 6:26 p.m.