I have some friends in the security field. There is a nice market for some aspects of it as well. Like government mandates on privacy of certain data. Like financial data and HIPPA.
You will want to know Operating Systems very well. You will need to learn how the IOS works on managed networking hardware. You will need to probably know some basics of software development. Web development will probably help too, since the rise in usage in CMSes is growing and growing and those things have so much code in them there are bound to be tons and tons of security holes in every major release.
You'll have to learn enterprise level stuff too, like Server OSes and such. Tons of security aspects in that as well.