You are not comparing like with like. The potassium in a banana is mostly passed through the body harmlessly, as only enough to maintain the normal level is absorbed.
Mostly correct. Instead of only absorbing "only enough to maintain the normal level", what you will actually get is absorption of a bit more than enough to maintain the normal level, coupled with increased elimination (mostly via urine) to maintain that normal level. Either way there is no difference -- there is no long-term storage of Potassium in the body, it is all present as the soluble, highly-mobile aqueous ion. So any increased level of from a radioactive source will relatively rapidly come back down to equilibrium levels of radioactivity, once you return to your intake from your regular Potassium sources.
Anyway, the ratio of radioactive Potassium (to non-radioactive Potassium) in your body will be equal to the average level of radioactive Potassium in Bananas (and other dietary sources, mostly plant-derived materials); the Potassium-40 isotope to non-radioactive isotopes is mostly at equilibrium concentration in the environment. For a 70kg human this means approximately 160g of total Potassium in the body, with 0.0187 grams of 40K, producing 4,900 disintegrations per second (becquerels).
The strontium in this water is absorbed by the body like calcium, accumulating in the bones where it will sit for years or decades slowly irradiating you, which is why is causes cancer and leukaemia.
Partially correct. Like Potassium, Calcium is regulated at a "normal" level, and the body will reduce absorption (from the gut), and increase elimination (mostly through urine) to eliminate excess. Accumulation happens if there is a deficit, or with active deposition of osseous material. However, due to constant turn-over of bone Calcium, at any given time a small amount of material is simultaneously being both absorbed and released from long-term storage. So this means a small amount of the ingested material will go into long-term storage, even when your body is not actively increasing Calcium stores.
However, note that while Potassium-40 and non-radioactive Potassium are chemically identical (well, almost identical -- some tiny kinetic effects may be present, negligible), Calcium and Strontium are not. They are grossly handled the same by the body, but there may be some differences in absorption / retention / excretion rates between the two substances -- so the radioactive Strontium will not be a straightforward constant fraction of the Calcium pool as it moves around in the body.
I'm disappointed. I expect more than this level of scientific illiteracy from +4 Slashdot comments.
I'm not disappointed; I never had any expectations to begin with :)