For me the path the heavy duty back-ends was Unix and C. Most of the work for large corporations, in addition to the mainframes, involves big systems; IBM: AIX

For me the path the heavy duty back-ends was Unix and C. Most of the work for large corporations, in addition to the mainframes, involves big systems; IBM: AIX, HP: HPUX, Sun: Solaris. Helps to know a bit about storage: EMC, Hitachi, NetApps, etc. And of course databases, DB2, Oracle. The best news is that these days, you can build up these skills using a $1k box with Linux or BSD. Years ago, you needed to get a job first because systems were in the order of $millions and they wouldn't fit in your average spare room. You'll also need to demonstrate so CS/SE chops, because mucking up a big back-end system is not like a web page that occasionally crashes, it can cost $10k's per hour while it's down.