With the explosion of the Fukushima nuclear power plant, many mushroom farms in the area have been affected. Japan eats a fairly large number of mushrooms per person compared to citizens of the states, so it is a significant issue for their population. Here is a quick look at some of the stats:
Normal radiation levels in Shiitake from Japan are 1.3 to 6.4 Bq/kg in fresh product and 6.7 to 73.9 Bq/kg in dried product (from 2001). The legal limit is 500 Bq/kg. A farm 250 km away from the plant was recently found with levels of 2,770 Bq/kg in their cultivated Shiitake. Due to these high levels of radioactive contamination, many of the outdoor farms in that region have been shutdown. There was even an indoor farm that tested above 1500 Bg/kg for their Shiitakes.
I looked briefly in the scientific literature and found a couple neat things. Average levels of Cesium are about 30 times higher in cultivated Shiitake than Oysters in Japan. But a study in Ukraine after Chernobyl found the highest levels in Oysters growing on Pine (Above 32,000 Bq/kg). As might be expected, levels in Shiitake are a function of Cesium levels of the rain and dry fallout. The levels also seem to be inversely correlated with atmospheric temperature. Shiitake grown in logs tend to accumulate more than those grown on sawdust blocks. (This is probably due to indoor vs. outdoor cultivation.) Finally, Matsutake accumulates more Cs than Shiitake, Grifola about the same.
There is only a limited amount of research that has been done regarding the accumulation of radioactive cesium into cultivated mushrooms. I only found about 5 or 6 papers on the issue. Much of the available information was already coming out of Japan, so expect to see a lot more research being presented in the upcoming years.
This is yet another advantage to locally grown produce. You can be sure not to get mushrooms that glow in the dark!