Article on ZMA from Wikipedia.org:
A study (Brilla & Conte 1999) was undertaken on NCAA Football players during a 8 week spring training program. 12 took a ZMA tablet each evening and 15 used a placebo. The results showed:
ZMA: 11.6% increase, Placebo: 4.6% increase
ZMA: 30% increase, Placebo: 10% decrease
ZMA: 3.6% increase, Placebo: 21.5% decrease
It is important to note that this study was funded by SNAC Systems Inc (the patent holders) and that one of the study's authors (Victor Conte) has equity in this company.
However, another study (Wilbornl et al. 2004) found that ZMA has no effect on strength or hormone levels.
Brilla LR, Conte V (1999). A novel zinc and magnesium formulation [ZMA] increases anabolic hormones and strength in athletes. Sports Medicine, Training and Rehabilitation Journal (): 585s-593s. 
Lukaski HC (2000). Magnesium, zinc, and chromium nutriture and physical activity. American Journal of Clinical Nutrition 72 (2): 585s-593s. 
Wilbornl CD et al. (2004). Effects of Zinc Magnesium Aspartate (ZMA) Supplementation on Training Adaptations and Markers of Anabolism and Catabolism. Journal of the International Society of Sports Nutrition 1 (2): 12-20. 
Aparently the recent study by Wilbornl found ZMA useless for athletes. Has anyone read the actual journal? Why has the study made ZMA seemed useless when so many people are reporting improvements in sleep/recovery etc...