Ed,
I understand where you are coming from in terms of names and phone numbers of people "out there" in the business world - people-data which you could easily re-generate on your own after leaving your firm. That data seems innocuous enough and its always hard to say that it is intellectual property.
But the original question, again, was about the ethics of taking an electronic file(s) from your employer upon leaving...not its legality. Is it morally right to take an electronic file from your employer when it was created by an employee of the firm (you), during work hours, and for the purpose of assisting the firm in better managing customers and clients?
You can make up all sorts of scenarios about "I did it on my own initiative after fulfilling all my work duties" - but that's just trying to justify the action. Its still unethical. As an employer, I could counter-justify you by saying, "If you got all your duties done, why didn't you come to me for more billable work instead of preparing that database?", or "Thanks for using your PAID for work time to create such a valuable database FOR OUR FIRM." You simply can't get around the fact that the company paid for that data and owns it.
I've got certain spreadsheets that I created on company time. They belong to the company since they paid me (my salary) for them, regardless of whether I was fulfilling my specific job duties. No one asked me to make them. I was the only one who percieved that they would be needed and valuable. But that doesn't change the fact that the company paid for them and owns them. I've got other engineering spreadsheets that I made at home and use at work - they are mine.
The electronic Outlook file that was created may have further data, beyond yellow pages info, and if so, that makes it even more unethical to take from the firm.
The company can always be asked, and give permission to take the data, which would then make it ethical. But without that knowledge and concurrence, its wrong to do it.