When you upgrade to SQL Server 2012 Integration Services (SSIS) on a machine that has SSIS 2005 or 2008 installed, SSIS 2012 is installed side-by-side with the earlier version. Unlike upgrades to previous versions, the upgrade process does NOT remove the SSIS 2005 or 2008 files, service, and tools.
In addition, with this side-by-side install, multiple versions of dtexec utility are installed. To ensure that you run the correct version of the utility, at the command prompt run the utility by entering the full path (<drive>:\Program Files\Microsoft SQL Server\<version>\DTS\Binn).
For more information about upgrading to 2012 SSIS, see Upgrade Integration Services and 5 Tips for a Smooth SSIS Upgrade to SQL Server 2012.
I noticed that myself too...
running WHERE DTEXEC from a command line tells me which versions are installed.
I preferred SSIS 2012 tools to be the default, so I adjusted my environment variable $PATH so that 2012 was first.
Running WHERE DTEXEC again shows me that the 2012 version is first (and hence the default)