How is the employment market for database management (DBMS)?
As long as there are databases and as long as people need structured data storage, there will always be a need for people to program/administer databases.
And which DBMS are currently most popular (Oracle, MS SQL) and employable?
Both Oracle and MS SQL Server are widely used. Mysql is becoming more and more popular for web applications.
What is the learning curve to learn DBMS from the beginning (for someone coming from a general IT background with a decade + old B.S. in MIS)?
Learning to write a SELECT statement is not rocket science. SQL is, at it's core, a very simple language.
A larger part of the deal isn't the language, but learning how to effectively and efficiently use the database. Learning how/when to index, how to optimize queries, proper database management, etc.
What other areas of IT are worthwhile exploring for training towards employment?