Seems Infiniti was not happy with the current numenclature and decided to change it in a way it relates to its roots and in particular with the Q45, but also representing a new brand direction.
In the image above you can see that the Q** will be used for the sedans and coupes, while the QX** is for all the crossovers and SUVs. From 2014 all models will have their badges changed and the first to get the new numenclature is the Q50, an all-new premium sports sedan to be unveiled at the NAIAS (North American International Auto Show) in January 2013.
Source: Infiniti
Source: Infiniti