Hi Spodman;
Sorry for the delay, I have been out of town. In the US a transponder is not required to fly IFR. However, it might keep you out of the busier airspace just due to the amount of traffic and the possibilities of not being able to use radar separation with you, especially enroute in class A airspace. But other than that, we handle non-transponder equipped aircraft as we can when we can...
regards
Scott