Unfortunately I can’t give you a precise response to your question cause as you said, nor really official documentation about this…
But I have experienced some issues about this point and I may have some usefull informations for your or someone else.
The facts :
I had a Magento v 1.1.8 release that was running right without InnoDB support activated on our dedicated server.
I recently made an upgrade to v 1.2.1 and somme errors appeared particularly on cart and checkout pages. This is the famous error taht everybody is talking about
See more at : http://www.magentocommerce.com/boards/viewthread/26356/
The debug :
I activated InnoDB support for mysql, and manually specified which storage engine to use on a per table basis, following this list(http://www.magentocommerce.com/wiki/groups/227/convert_database_to_innodb).
Add to cart and checkout process seems to work again properly now.
Some hints :
But there is still some “not so clear” points. Maybe InnoDB became mandatory since 1.2.* versions.
Anyway after activating InnoDB support, I tried to make the magento install process starting with an empty database( in order to check which tables Magento was expecting to use InnoDB instead of MyISAM). The result is so poor…
All the tables were created using MyISAM and not InnoDB, but if you run the installer without InnoDB activated, at the database setup step I got an error telling “You system doesn’t support InnoDB.”
Please Magento Core Team, tell us more about his ?
What is the goal of making InnoDB a requirement if your process install doesn’t create any tables using it ?