[eluser]BrianDHall[/eluser]
To fully 'fix' the issue, delete your whole application directory and install the newest version of CI. Then you can copy over your customized files (config, controllers, etc) and your error should go away.
CI 1.7.2 supports php 5.3, but 1.7.1 uses features that became officially deprecated as of PHP 5.3, as Kip points out.
I had a similar problem to you when I tried to upgrade CI version, not sure what caused it but fully deleting, reinstalling, and copying over my files fixed it.
