As bikes continue to get stronger and stiffer and lighter, some of the standards evolve and change. One area of constant evolution is in the area of headsets. This page will help you determine which headset will fit your Ibis.
A group of manufacturers recently came together to create a standardized nomenclature for headset naming. Getting bike manufacturers to agree on something is a little like congress coming "across the aisle", except this actually happened!
Here's the website they've created: http://bicycleheadsets.com/
The headset naming protocol goes like this:
And here are the naming conventions for our headsets: