Have you actually seen any arbitrage opportunities lately? I've only been watching Mtgox and Tradehill, but the two have moved in rigid lockstep for maybe a month or two now.
I am seeing something similar actually.
While this might also be a coincidence since there haven't been any huge moves in the BTC price lately (which usually means fewer chances for arbitrage),
I just realized that my last trade was over a week ago on the 27th of last month, which is only a few days after the release of my arbitrage program (the 24th). And prior to the release it was still finding trades pretty much every day, and often multiple trades each day.
My results are using three different exchanges and not just the two you mentioned, but it is telling a similar story.