Two reasons to consider joining the mainframe workforce:

 1. It pays well
 2. There are tons of openings

The greying workforce in the mainframe field is, and will be creating **huge numbers of openings** in the field.  

I work for a large financial company, and within the next 5 years, we'll be losing about 30% of our workforce to retirement.  That number will increase exponentially in 10-15 years.

More reasons:

 - I've been in the field for 25+ years and have never been bored.
 - Less competition for jobs.
 - Stop complaining about the technology (see some posts above)... it may be old, but in many ways it's light years ahead of open systems.  HTML - give me a break.  It's so similar to Basic which I took 30 years ago in college.  We're way beyond that.
 - The mainframe is fast and reliable, tried and true.
 - Try Systems Programming if you're very bright and love trouble shooting.
 - As a team leader, I wish I could find young, trained technicians to fill openings. 
 - Did I mention it pays well?
 - Other mainframe career options besides software development - hardware engineers, storage techs, networking, and more. 
 - It's fun, exciting, challenging, and there's great career growth.
 - Stop thinking of mainframe as just old technology - check it out and verify all that I've said.

Also check out IBM's System z Academic Initiative.