Today I faced some problems running SSIS 2008 packages on a server where SSIS 2008 and 2012 was installed.
The SSIS 2008 binaries are placed in the 100\DTS\Binn folder and the 2012 binaries in the 110\DTS\Binn folder.
When clicking a DTSX file it is hard to see which DTEXEC version will be used.
When running dtexec.exe from the commandline you can check which version is actually used by default.
Microsoft (R) SQL Server Execute Package Utility
Version 10.50.4000.0 for 64-bit
Copyright (C) Microsoft Corporation 2010. All rights reserved.
Version 10.5 is the SSIS 2008-R2 version. 11.0 is SSIS 2012.
Solution for running the SSIS 2008 version by default
Remove the PATH environment variable contaning the following string: \110\DTS\Binn\
Be sure the PATH variable contains the 2008 SSIS binary path : C:\Program Files\Microsoft SQL Server\100\DTS\Binn\
Of course you can do this trick the other way around if you want to use SSIS 2012 by default.