as long as there is IT business there will be MAINFRAMES!
Most projects on MAINFRAMES are maintenence, or minor upgradation. There are some where major upgrades are needed. Example Y2K. It did not need anything brilliant to solve that. But you needed many people to code and test the changes.
MAINFRAME as an IT career is good. You will never have a gap in your career. although you may not like the type of work you do. (in most cases).
IBM is pushing MAINFRAMES very aggresivly, they have brough LINUX on to the OS that runs MAINFRAMES, ZOS. lots of cool fundas are there, but you need not worry about much of the fundas

99% if ppl working on mainframes (includes me) might not have seen a mainframe!
regarding training, yep MAINFRAMES, youmight need training, cause you cant install ZOS on your home PC. However, looking at the current recuiretment scenario, if you get placed in a decent company, then the company will provide all the training you need. You dont need any additional training before joining a company. (It will be sheer waste).
XPEDITER.
ps: xpediter is a tool for debugging programs on mainframes.