Pidyon shvuyim is an absolute mitzvah, which must be followed at all times. There is ONLY one major exception, as explained in the Gemora (Gittin 45a). And that exception is either (its a machlokes) a-its too big a financial burden on klal yisroel or b-they goyim might take more Jewish prisoners. That is the ONLY exception.
Joseph, I always heard that if they are going to be tried fairly (such as we have in the US), than pidyon shvuyim is not mandatory. Do you know if there any source for that?