Depleted uranium has two possible modes of instigating biological damage — ionizing radiation due to the fact that it's a radioactive metal, and biological toxicity due to the fact that it's a “heavy” metal. Regarding the first of these, radioactively “depleted uranium” is basically as little radioactive as it's possible to be and still be radioactive and not inert. This may sound like a quibble, but the half-life of uranium-238, the major radioactive component of depleted uranium (since it's been “depleted” of other uranium isotopes) is 4.5 x 109 (i.e., billion) years (not "109" years as news pieces have erroneously reported). In other words, over the entire 4.6 billion year age of the Earth, the quantity of uranium-238 on this planet has decreased by only half. That is barely detectably radioactive at all, on the human timescale.
Even when it does decay, virtually all (> 99.99%) of uranium-238 follows the mode of alpha decay (emission of a Helium-4 nucleus), which cannot penetrate beyond a couple of inches in air and is stopped cold by sheet of paper. Contrast with gamma rays (high energy electromagnetic radiation emitted by some radioactive decayers) which can penetrate through feet or meters of lead and are highly destructive to biological tissue.
The possibility of heavy-metal toxicity by uranium is potentially of greater scientific import. That, though, is fundamentally no different than toxicity due to say lead, which has traditionally been used (without environmentalists' extraordinary complaints) as bullets on battlefields for centuries.
And from the comments:
In a number of applications depleted uranium is used specifically to shield people from radiation.
Ordinary drywall dust is more radioactive than DU. Window glass is more radioactive. Many phosphate fertilisers used on crops are more radioactive.
Ignorant, silly, agenda-driven ninnies, none of whom has a clue that 'lamda' signifies anything other than a homosexual advocacy group.
Depleted uranium could be scattered all over my farm and I would nevertheless be at greater risk from mould spores growing on the depleted geraniums [insert rimshot sound] in my greenhouses.
Can't confirm any of that comment, but radioactivity paranoia is all around us. People still whine about global warming but won't hear of constructing nuclear plants while the coal-burning power plants down the street put out enormous amounts of radiation and they don't give a crap. Nimrods are forever up in arms with no real basis in reality; it's one of the pleasures of being part of the human race. But enough with the DU paranoia (hey, that's a pun in itself - Democratic Underground is nothing if not paranoid).