We hire rigging interns here and in general the bar is not too high.
First I need you to know Maya well and be able to trouble shoot an issues or perform basic rigging without a lot of mentoring on my side (simply not enough time in a day for that)
Also I need you to be able to automate stuff using a scripting language (nothing fancy) Python preferred!
And finally familiarity with a linux environment and svn is a plus.
The rest is on the job training.