Actually the meaning hasn't really changed at all. The original "hackers" broke into computers and networks to play pranks on their collegues in the AI lab. Later "hackers" hacked the phone system to make free long distance calls. I find Paul Buchheit's definition of "hacking" the most accurate: find and exploit differences between the perceived rules and the real underlying mechanisms of a complex system. This can be perfectly legal in some cases, and illegal in others. It is all hacking nevertheless.