Red Hat Software's Certification Program
Red Hat is in the process of creating a "certification program" so as to encourage the construction of a "network" of consultants that support their version of Linux. Here is their Red Hat Support Engineer job description
There have been some criticisms that they have acknowledged in the white paper Linux Certification - Pros, Cons and Directions
AIX Certification - Online Test
Might be worth looking at to compare to possible Linux certification programs...
www.jcinc.com Jeffersonian Consortium - offer North Carolina training courses including re Linux
Who vend test simulators for a variety of certification tests including Red Hat's RHCE