Of course there is the 'knowing you're being reverse engineered and doing something else'. I don't doubt for a minute people who write sensitive code, be it malware or DVD decoders, might simply act differently if they thought a debugger was involved, not so much as not act at all. Some of these techniques could be used there.
That being said, the more interesting thing is poking around in the inner bits of the machine and seeing how it comes together. Highly recommended for anyone serious about wanting to know how the machine does what it does.
If you want to practice on code that is easily obtained I suggest you poke around the World of Warcraft rootkit code that it uses to prevent people from cheating at WoW.
Detecting debuggers and altering behavior is so much the oldest trick in the book that it is actually covered in depth in this Codeproject article (Codeproject is often unusually well written, and so is this article, but be clear that this is really basic stuff he's talking about).
That being said, the more interesting thing is poking around in the inner bits of the machine and seeing how it comes together. Highly recommended for anyone serious about wanting to know how the machine does what it does.
If you want to practice on code that is easily obtained I suggest you poke around the World of Warcraft rootkit code that it uses to prevent people from cheating at WoW.