While the speculation has been rampant in the community and Obsidian has kind of answered the question, it is now official. Here's what J.E. Sawyer had to say:
Rideable horses will not be in NWN2. I'm sorry it's taken so long for you to get that answer, but there it is.
A lot of components of horses are in the game right now, but bits and pieces don't make a working whole -- especially when that working whole is a vast sprawl of interconnected subsystems.
Given their partial implementation, the large amount of work remaining for them, and their low integration into the official campaign, we decided to cut them. My apologies to those who may have been depending on mounts for their NWN2 module ideas and for players looking forward to using mounts in the OC and enduser campaigns.
Additionally, when asked if the material originally created and then discarded would be left in the toolset for modders to use, Sawyer answered:
At best, incomplete animation sets, sketchy mechanics, horse models, some horse accessories -- things like that.
Read the thread here.
