I learned a lot by doing the coursera course on it earlier this year including an intro to a whole new area of mathematics. Some sort of intro to crypto techniques and uses really ought to be mandatory for a lot of devs, certainly anyone tempted to use anything other than a well-coded TLS API or pre-provided GCM interface.

--edit--

In fact it's also very important that even when you do know what you're doing (to a greater extent than the entirely uninitiated), you don't implement this stuff yourself, because there are more attacks than you can possibly imagine.