Reddit mentions of Discrete Mathematics and Its Applications

A lot of it is probably confirmation bias, but yes, it does happen.

HP used to have expiry dates on their cartridges claiming they degraded printing after a certain time: http://www.hp.com/pageyield/articles/uk/en/InkExpiration.html

Another example from the software development world: Red Gate announced that one of their products (Reflector) would no longer be free starting from the next version and disabled all existing free copies, a move that upset many developers: http://www.infoq.com/news/2011/02/NET-Reflector-Not-Free

College textbooks are the most literal example of planned obsolescence; the new editions often contain very few new material and cost a lot while all older versions can be bought for almost nothing... and of course most classes require the new version.
For instance, Kenneth Rosen's "Discrete Mathematics and its Applications" currently sells for $125 if you want the [latest edition] (http://www.amazon.com/Discrete-Mathematics-Applications-Kenneth-Rosen/dp/0073383090/), $100 for the one before that and $16 for an older one even though the number of pages only increased by 100 each time. Thankfully, my teacher gave us the page numbers for both the latest and the second-latest editions...

You need to show that you know your stuff. Just because you're doing something more applied like Network Security in grad school doesn't mean that you won't have a base level of knowledge you're expected to understand. In that case, you need to learn some basic stuff a CS student at a good school would know. I'm not "dumbing down" anything on my list here, so if it seems hard, don't get discouraged. I'm just trying to cut the bullshit and help you. (:

• Redo your introduction to Computer Science. If you finish this, picking up a new language is cake.
  
  
• Discrete Mathematics, A.K.A. "Math for Computer Scientists" This is the standard text for this, but this is pretty good for a cheap book.
  
  
• Algorithms
  
  
• Compilers
  
  
• Operating Systems
  
  
• Networking
  
  
• For basic CS theory, "Introduction to Theory of Computation by Michael Sipser" is what I used to recommend, but Amazon doesn't seem to have a sanely priced copy. Either buy that used, or get the classic "Cinderella Book". Get an older edition if you can!
  
  Again, don't be discouraged, but you'll need to work hard to catch up. If you were trying for something like mathematics or physics while doing this, I'd call you batshit insane. You may be able to pull it off with CS though (at least for what you want to study). Make no mistake: getting through all these books I posted on your own is hard. Even if you do, it might be the case that still no one will admit you! But if you do it, and you can retain and flaunt your knowledge to a sympathetic professor, you might be surprised.
  
  Best of luck, and post if you need more clarification. As a side note, follow along here as well.
  
  Netsec people feel free to give suggestions as well.