The rumors have been circulating for over a year now at RCGroups and various heli forums like RunRyder and HeliFreak about how Spektrum would be replacing DSM2 because of it's issues working at busy flying events like Joe Nall where many pilots were getting locked out using DSM2 radios. Well they have finally admitted to a certain degree that DSM2 has issues with 'busy' flying fields.
DSMX is their solution to a more reliable connection to better compete with Futaba's FASST and Hitec's Aurora 9's FHHS system.
DSMX is backwards compatible with DSM2 RX's. So if you upgrade your DX8 (software update) it will still support your DSM2 receivers. DSMX receivers will also work with DSM2 transmitters like the DX7 or DX6i. The JR9503, 11X, 12X, DX7 and DX6i can be upgraded for DSMX ($75 fee).
For more information read here: