0x41414141 associated to security exploits?
It's nothing fundamental. It's just a historical convention, like using
foo as the name of a variable when you have no clue what to name it.
In more detail: The simplest way to test for a buffer overflow is to type a long string of A's (AAAAAAAA...) into a text field, and see what happens. If the program crashes, it might be vulnerable. If the program crashes and a debugger shows 0x41414141 in the program counter, ooh boy, you hit pay dirt: the program is almost surely vulnerable. (Remember, the ASCII code for 'A' is 0x41 in hex, so 0x41414141 is what you'd see if you looked at the byte-level representation of a string of A's in a hex editor.)
Why A's? No reason at all; they're just the first letter in the alphabet.
So, this is a quick-and-dirty test that pentesters sometimes use. But of course, there's nothing special about 0x41414141. Douglas Adams fans could type in a long string of B's, and then look for 0x42424242. That'd be equally effective, and even more fun. I gotta remember to use that one in my next hacking demo.....