I feel the need to record a dissenting opinion on this.
It is my understanding that acknowledgments are not a required component of a PhD thesis. They are a tradition and a custom, and like most traditions and customs, people participate in them because they find them valuable and enjoy doing them, not because they are obligated to do so. One answer says that many grants "contractually require you to acknowledge the funding entity". Yes, but that's a different kind of acknowledgment: for instance, in many published papers that goes on a footnote appearing on the first page, whereas there may (or may not) be a paragraph in the introduction or at the end of the paper thanking various people. Moreover, a PhD advisor is not a "funding entity" in this sense, and I have never heard of anyone being contractually obligated to thank their advisor. Let us assume that the OP is not.
After reading advice that amounts to "Yes, you should absolutely write acknowledgments in a way that conveys a positive impression of your advisor. Since you in fact feel exactly the opposite way you will have to write them very carefully indeed" a few times, I got a bit worried. I see a personal integrity issue here: really one should not record in a formal document sentiments which are diametrically opposed to those one actually holds. Moreover, all the comments about the career risk one might incur for not including acknowledgments -- well, I must agree that omitting acknowledgments is a suboptimal career move, but such comments make me even more worried. It seems to me that many people here are essentially viewing acknowledgments as a loyalty oath that one must make to one's thesis advisor. That can't be good. People stood on principle against loyalty oaths in the past, sometimes with cost to themselves. They were right and courageous to do so. I'm afraid I see a similar principle here.
My point is this: no, actually the OP certainly does have the right not to put acknowledgments in his thesis. Is that the best career move? No, probably not, but it is his choice and he may have good reasons for doing it. Four final comments:
No acknowledgments at all seems much better to me than acknowledgments which are sarcastic or show bitterness, and probably even better than acknowledgments which are carefully written to extract exactly the level of thanks that a dissatisfied person can muster. I do agree that it is less than plausible that the OP does not have warm feelings about anyone (e.g. his family and friends), but writing acknowledgments to your thesis and not mentioning your advisor is also worse than not writing them at all. If you really want to thank your friends and family then you don't need a page in your thesis to do so. (I looked back at my own thesis acknowledgments just now and was somewhat pleasantly surprised by the effusiveness with which I thanked my mother and a very supportive ex-girlfriend. But now I have a sinking feeling in my stomach: I'm not sure either of them ever read my thesis! A facebook message may be in order here...)
In case people are wondering whether I personally identify with the OP's ingratitude to his advisor: hell no. My first reaction to the post was to roll my eyes at the sentiment that his advisor was helpful for half the time and then so unhelpful so as to make the total contribution 0%. I am struggling to imagine what someone could do to make a contribution so negative as to cancel out years of support. My first impression of the OP was that he is -- to put it mildly -- a peevish ingrate. My second thought? Gosh, this guy would make a much better impression simply by not talking about his advisor at all. Not recording for posterity your own sentiments when you know that they contain inappropriate and unseemly negativity is a sign of great professionalism, more so than just lying through your teeth.
I recently read a PhD thesis from my own program without acknowledgments. I must admit that I did notice this: when I saw the question I thought "Wait, didn't so-and-so not have acknowledgments in their thesis?" so I looked back and confirmed that it was true. Without giving away personal details, let me say that I have every reason to believe that so-and-so had an unusually positive relationship with their thesis advisor, and that so-and-so has gone on to another academic job and, apparently, a promising career. Why did so-and-so not include acknowledgments? I simply don't know. It's not my business.
In contrast to what some other people have said: other than in the context of my own graduate program, I rarely read people's PhD theses. When I do it is usually to get some exposition or technical detail that I wish they had put into their published work. In particular, I almost never read the theses of job candidates. Does this sound weird? I think it isn't: I read lots of other accounts of the material in candidates' theses: from their recommendation letters, from their cover letters and research statements and -- when I am really interested -- from the papers and preprints resulting from their thesis work. If other people here specifically read PhD theses in the context of academic hiring, I would be very interested to know. Also, as a hirer -- yes, times are tough and we can be very selective, but there are certain things that one does not want to take into account. If I was at a hiring meeting and someone brought up a lack of acknowledgments in a PhD thesis as a point against a candidate, I would say something like "This person really must be great if you want to sink her and can't do any better than that."
