Unfortunately, unlike "windows" there's not just a "unix" Unix is a general term for many OS's. Linux is based on Unix but itself is not a Unix, OpenBSD, FreeBSD,NetBSD are all Unix based but again aren't true Unices... AIX (IBM) HPUX(HP) SCO (Sco group) IRIX (SGI) Solaris/SunOS (Sun) are the big name Unices you'd want to learn. However, you don't need to learn each individual OS in order to become proficient in it, most stuff on Unix/Linux is pretty cross platform ... so the "ps" command works automagically the same across the board in most instances..
And unfortunately, there's so many different companies who run different flavors of Unix, that its hard to say which to learn first. In my expererience my top 3 would be Solaris (big with ISP's) AIX and HPUX (data houses ect) ...
Unix itself was more or less born out of AT&T during the 60's and much of the code that was used then was licsensed for use in SCO Unix
AS/400 as someone suggested would be a good thing to have under your belt tho, however its an IBM Mainframe platform and so its not really "unixy" so to speak... and from what I've been told, mainframe's are about as much fun as having your eyelids pecked out by slow eating pigeons paying homage to calista flockhart..
I'd say learn Linux... or if you know Linux, learn it better
and concentrate on the after market packages such as Oracle, Postgre/MySQL DBA stuff.. Tivoli and OpenView on Unix .. stuff like that
Hope this helps and sorry to everyone for rambling on.