The fine folks over at Kotaku have uncovered a job ad that seems to indicate a serious new game in the HOMM universe is under development.
The advertisement, posted on Gamasutra by UbiSoft Studios in Shanghai, originally said that the studio is developing "Heroes of MM" as "a brand new fantasy RPG title for the PS3, Xbox 360, PC, and Mac". The original job ad has since been edited, but the job still calls for someone who has "shipped at least two AAA RPG titles in a senior or lead game designer."
Being an "RPG" game means this could be HOMM6 or a whole new adventure/RPG. At this point I'm leaning towards the latter given that turn-based strategy games don't normally end up on the Xbox or PS3 (or the Mac -- haha). It might also make sense given that UbiSoft Shanghai has been responsible for creating titles like Splinter Cell: Pandora Tomorrow, Ghost Recon Advanced Warfighter 2. On the other hand they did a pretty good job with the strategy RTS title EndWar in 2008, so this could go either way.