I am seeking to move from Mainframe Storage Management to Open Systems, possibly as an AIX administrator. Does anyone know of a good Unix certification program that can get my foot in the door?

Solaris certification is easy, AIX is easy too both take some 1 month to prepare to with a right book. You can attend 4 days AIX training and then pass some simple exam to certify as beginner administrator (AIX SA 1 if I remember it right), if need the initial training, as I mentioned for many times, get "UNIX Essentials and UNIC Core" DVD (it comes with certification) and Evi Nemeth's "UNIX SYSTEM Administrator's handbook" it is a bible. Have in mind that AIX pays better than Linux and as to I is much pleasure to work with. I do AIX since 1994 and it is GREAT system.

Are instructor-led classes required for UNIX Certification, and how can I find out info about locations and dates of the classes? Or can I get all the training online?

For Solaris see sun.com for aix ibm.com,
there's no requirements for class to take exam, you can learn stuff by yourself and then go to any of countless locations and take the exam. If you pass the certificate will arrive by mail. I think AIX those days requires 4 days training but I'm can be wrong about it.