I would be surprised if its not already underway. Formal testing takes significant amounts of time and is hence often coordinated with releases of major versions of System 800xA.
Please ask your regional ABB sales representative for help with forwarding a question to Product Management.
ESXi 6.0 is not yet officialy supported for 800xA. Latest version of 800xA ie 6.0.2 Supports ESXi 5.1 and 5.5. Same shall be the case with immediate next 6.0.3 version of 800xA. Support for ESXi 6.0 will officially come probably in next major release of 800xA.
Contact regional ABB sales to know exactly when.