Brian, interestingly enough my docs (both urologist and radiation oncologist) wanted to wait a full 6 months post brachy before doing a PSA. I was Iodine also and that was part of the logic. At the 6 month time they wanted to see the PSA cut in half before they would be "delighted". As JT said it can take years before you reach your nadir and as long as it's heading in that direction you should be fine. I have read at least one study (that I can't put my hands on right now) that quoted the median time to nadir as 40 months. Obviously then it could be much longer.
I think your PSA test might have been a bit early but the PSA is heading south and that's what you want. Continue to enjoy life and assume the best...let us know at 6 months...chances are you'll continue to drop.