I had the impression that obtaining written permission before including someone in acknowledgements was required. Therefore, I just wrote to someone asking for written permission to include him in the acknowledgements of a paper, and he said he didn't think it was necessary. I don't recall where I got this notion from - perhaps the rules of a specific journal? So, I was wondering if there are any general rules about this or not, or are they perhaps journal specific?
When you list someone in the acknowledgments, you're just thanking them, as opposed to speaking on their behalf or assigning them responsibility (the way authorship does), so I don't see why permission should be required. I've never asked for permission or been asked myself, so it's certainly not standard in mathematics. I haven't heard of it in other fields, but of course I can't say from personal experience.
Of course it depends on what you say. "I am grateful to Alice for her steadfast support of my research" suggests Alice endorses your research, and you should certainly ask for permission before saying something like that. "The determinant calculation in Section 2 was supplied by Bob" suggests Bob is responsible if it's wrong or clumsy, so you should make sure he is OK with being thanked (but in this case you presumably already discussed with him your plans to include his calculation in your paper and attribute it to him without making him a coauthor). And of course if your topic is really controversial, then you should be extra careful about everything. However, if you had helpful background discussions with Carl and write "We thank Carl for helpful discussions about functional analysis", I don't think you need to ask his permission.
While Anonymous Mathematician’s answer holds for most fields I am aware of, this question and answer made me aware that some journals in the field of medicine and some mega-journals require consent for being mentioned in the acknowledgements, for example:
I've never come across any rules about this. But I do often write to people to let them know I'd like to include them in the acknowledgements, particularly if I know them less well, and enclose a copy of the draft paper. It's a way of thanking them, especially as they might never come across the published paper, and it gives them a chance to escape if, for whatever reason, they wish to.