A recent study by Goodin et al. found that 56g of soy protein powder per day caused serum testosterone to fall 4% in four weeks in a test group of twelve healthy males. (Note that there was an error in the abstract that said 19%, and this figure was erroneously reported by the media. The article reports 4% and not 19%. See the following Web site for a more complete analysis of this study and the error in its abstract: http://findarticles.com/p/articles/mi_m0KFY/is_9_25/ai_n27410483.) According to the study, this data supports further studies of these hormonal effects as a mechanism in prostate cancer prevention. [20] However, a study conducted by the Miami Research Associates refutes the finding of the Goodin study, finding that soy protein had no significant impact on testosterone levels in healthy males. [21] In fact, only one participant in the Goodin study actually saw a drop in testosterone. The participant in question had testosterone levels 200% higher than all of the other subjects, and during the study, his levels dropped to bring him in line with the other participants. The Goodin study did not conclusively suggest that the participant's erratic testosterone levels were related to the soy protein.
I think either way, it makes no difference unless you are consuming a massive amount of it, which may make no difference anyway.
I hate the taste of Soy Milk and generally avoid soy products (except soy sauce). Sometimes they put soy protein in energy bars or protein shakes, which doesn't bother me as long as it is just a portion of a mix of whey and casein.