You don't have to have a Linux certification to get a job working with Linux, but it can't hurt.
Simply working on Linux is easy. Join a Linux community like The Debian Project, Red Hat's Fedora, Novell's OpenSUSE, or half-a-hundred projects and go for it.
Of course, you won't get paid, and 95 percent of the work on Linux projects is programming. So, if you want a salary, and your skills lie in system administration or network management, it behooves you to look into Linux certifications.
Personally, if I will have time, I want to take an LPI Certification: it's really "vendor-indipendent" (I see some of the tests) and various company recognize it.