As of this moment I've just renamed the `sideblock` class that was being applied to all blocks to `block`.
I've updated all core themes now but those of you who are working on or with your custom themes will need to update your themes CSS when you next update.
.sideblock has now become .block
This was done after Martin and Patrick came to the conclusion that sideblock was misleading in that now the theme can put blocks anywhere.
MDL-22358 was created to track this issue.