In fact, Ubuntu as many other Linux distributions have switched to udisks2 which is used for the auto mount feature.
In the new version of udisks, the default mount point is
/run/media/$USER, but it has been patched by Ubuntu (and some other distributions) to be
/media/$USER (See this answer from Florian Diesch for more details).
udisks version 2.0.0, which is included in Ubuntu 12.10 doesn't allow you to change the default behaviour which is mounting a file system as non-shared (only accessible by $USER).
Since udisks 2.0.91, it is possible to change the default behaviour so that mounting a file system as shared can be done as before [
/media] (See this answer from rocko for more details).
The root cause for this change of default behaviour in udisks2 seems clear : the security. It is safer to restrict access to a file system to one particular user instead of giving access to it to all the users of the system.