That's a good question. One that I have considered my self many times. My opinion is that Omega-3s in reasonable doses don't interfear with growth from training. Here is my reasoning.
1) Omega-3s don't inhibit PG production like NSAIDs do.
2) Omega-3s have been found to favorably effect both insulin sensitivity as well as inhibit protein breakdown. (Sohal PS, Baracos VE, Clandinin MT. Dietary omega 3 fatty acid alters prostaglandin synthesis, glucose transport and protein turnover in skeletal muscle of healthy and diabetic rats. Biochem J. 1992 Sep 1;286 ( Pt 2):405-11.)
Finally it is important to keep in mind that too much Omega-3s is not good. It's not natural either. The beneficial effects on insulin sensitivity and inflammation can be acheived with intakes of 2 grams per day. This is considerably lower than what is used in animal studies looking at the effects on PG production.