Not so fast....
Plutonium's toxicity is much less than commonly stated. Long term studies of people who have been exposed to plutonium either in weapons manufacturing, fuel reprocessing and bomb detonations have found no excess deaths due to its toxicity. It is most certainly not the poisonous substance on Earth as Ralph Nader hyped it, it may in fact be less toxic than caffeine.
Plutonium is an alpha emitter, and in theory can produce a wide range of cancers if ingested or inhaled; but once again, long term studies of people exposed to plutonium particles have failed to show a significant risk.
However, plutonium in the environment is a sign that other nasties which are biologically active and have much higher radioactivities are probably also slopping around in the outside world.
The Pu-238 found at two locations at Fukushima is almost certain to have come from the reactor rather than a bomb test because Pu-238 is only likely to have been made in a reactor by one of two processes:
U-235 + n + n -> U-237
U-237 beta decays to Np-237 over a period of days
Np-237 + n -> Np-238
Np-238 beta decays to Pu-238
There is no time in a bomb explosion for significant numbers of intermediate neptunium to be created and to undergo beta decay so Pu-238 is a good indicator of reactor plutonium.
The second method for making Pu-238 is for Pu-239 to be hit by a fast neutron that rather than fissioning the nucleus ejects a second neutron. This can happen in bomb detonations, but it will not create significant amounts of Pu-238 to affect the isotope ratio.
Clearly there is a leak either in the reactor itself or in the spent fuel pond.