Appleton et al. has published a recent review of evidence regarding the psychiatric effects of omega-3 supplementation.
Here's the link:
Basically, the conclusion is similar to my previous impression on this issue: there is more evidence coming out, generally supporting the possibility that omega-3 supplementation can be modestly beneficial for treating depression. But the existing evidence is somewhat shaky, heterogeneous, and probably influenced by publication bias.
The authors overstate some of the conclusions: for example, they claim that, based on the evidence, omega-3 supplements are unlikely to be useful to prevent depression in a healthy population. This is unfounded, since there were really no adequately long studies which aimed to show preventative effects.
Another of my usual complaints about the studies described is that they are of inadequate duration: many lifestyle changes or treatments that could affect depression (an illness with a periodicity which is often over years or decades) may require several years of disciplined adherence before significant benefits would become apparent. Most of the studies described were less than 3-6 months in duration.
Another study by Amminger et al. from the February 2010 issue of Archives of General Psychiatry (http://www.ncbi.nlm.nih.gov/pubmed/20124114) assessed subjects with signs of early psychotic disorder who were randomized to receive 4 capsules per day of fish oil (containing omega-3 fatty acids), or placebo daily, for 12 weeks. In the following year, substantially fewer individuals in the fish oil group, compared to the placebo group, went on to develop ongoing psychotic illness (5% vs. 28%).
I do encourage omega-3 supplementation, as it poses negligible risk, with a modest potential benefit, both with respect to mood and to some other areas of health.