As I understand the certification universe, there is no "Ubuntu-specific" certification. Mainly, the general universe of execs hiring IT Professionals don't look for just "Ubuntu Certification", more often they're looking for Linux expertise rather than just one specific OS. Sometimes they want expertise in the specific OS, and in fact in one workplace I know the sysadmin is CentOS/RHEL experienced, and one of the other staffers is Debian/Ubuntu experienced, and they both share linux administration, not to mention teach each other things from time to time.
If they're looking for a Linux certification, they're looking for a general certification. Here's some of them. This is NOT an all-inclusive list:
While each of these certifications has their quirks and difficulties, these are some of the most popular certifications I have seen.
The Linux+ certification is a nice starting point. So is the LPIC-1. If all you're after is a certification to say you're competent as a Linux sysadmin, that's probably all you'll need to be considered a server administrator.
The more specialized certifications will indicate you are a more global Linux Professional, but usually that's not necessary if you're after just a plain old sysadmin position, provided you can demonstrate ample knowledge to back up your skills.
Other certifications not in my list