The Eye-One iO Quick Start Guide states the following cleaning instructions:

The plastic housing of the instrument may be cleaned with a damp cloth with some soap.  Clean the white reference with isopropyl alcohol on a soft clean cloth. The electrostatic mat of the iO should be cleaned with Isopropyl alcohol on a clean cloth. Alkaline solvents and soap should never be used to clean the electrostatic mat.

The Quick Start Guide can be accessed at:

http://www.xrite.com/documents/literature/en/i1iO_QuickStart_en.pdf

If the mat needs to be replaced, it will need to be returned to your local X-Rite Service Center.