I have a Ph.D. in CS, and although I am highly respected by my colleagues, and currently involved in interesting projects, it did not help me much when looking for work. During various job interviews I got the impression that years of experience in the software business is considered more valuable than a Ph.D. When I did get hired, the interviewer actually had a Ph.D. himself, so he did see the value. If you look at CS job descriptions, a Ph.D. is almost never a requirement, except for university positions and some research positions, but those are scarce (in the current economy). So, a Ph.D. is no guarantee for an interesting job. There are lots of Ph.D.'s out there doing work that they are overqualified for.
A Ph.D. is really a training program for an academic career, not for a business career. The subject of a Ph.D. is often highly specialised or even obscure. So, unless you happen to know (and if you have a choice for a Ph.D. subject) what research area will become important at the big software companies in three or four years time, the subject of your Ph.D. is irrelevant, or even detrimental, for your career. (That also goes for university careers: its very difficult to fight your way into a different specialisation).
That said, I loved my time as a Ph.D. student and post-doc researcher. You should only do a Ph.D. if you are passionate about a subject. So, if you have the opportunity to do a Ph.D. and you can afford to do it, and you are inspired by the subject and driven enough to finish it, then go for it. But it's not necessarily a good career move.