Does posting a preprint to the arXiv reduce the chances to acceptance?
I'd say the answer will depend on the field. In maths it is hard to find new papers that were not posted to arxiv first. Disregarding or even disallowing such papers would spell death for a journal. Thus, regarding acceptance there are no pros and cons, it will simply not matter. Regarding all the other aspects, such as visibility, etc., publishing a preprint is an obvious choice, but that topic has been discussed extensively in previous questions.
I guess the same will hold true for physics and CS. It might be different in other fields where arxiv and similar sites are used less often, but it seems to me that there is a slow convergence towards extensive use of preprint repositories in all branches of science.