Shooting with a Nikon D3S at ISO's above 25,600, the EXIF data reports 36,000 no matter what the actual ISO was when the shot was taken
I don't have ACR 5.6, but the DNG converter converts ISO 102400 to 36864; ISO 51200 should be correct. The reason is, that 102400 is the first full stop ISO over 65535, the limit for an USHORT field value. Time to change the declaration to ULONG.
However, there is a big problem: while the ISO specification in the NEF file is in Nikon proprietory format and it can grow for a long while, the ISO in the Exif format is unsigned 16bit value, it can not be expanded; a new field needs to be defined (expanding the existing field may cause havoc under older programs).