Tantalum, a refractory metal, is an exceedingly rare mineral, mostly mined in Australia. It's used in the creation of tantalum capacitors , an extremely reliable sort of capacitor, which means you can find it in many of the most high-profile electronic devices, including those from Apple  and Intel . Though the majority of the world's tantalum mining takes place in Australia, the Congo is, according to the Enough Project, home to about a fifth of the world's known tantalum supply.
The Congo is also home to one of the bloodiest, ugliest, and most horrifying conflicts on the planet, and has been for years. Death toll estimates are well into the millions, as much as 45,000 per month by 2007.
A group calling itself the Enough Project has taken it upon themselves to try to alleviate the situation in the Congo through protests here at home. By purchasing Congolese tantalum, the Enough Project says, American corporations are indirectly financing the Congolese warlords and terrorist groups, including the FDLR , who perpetuate the killing.
Manufacturers like Apple and Intel claim they do not use tantalum from the Congo, citing assurances from their suppliers. But the Enough Project says this is not a reliable source, and that these companies are neglecting to check the supply at smelters due to cost concerns. The Enough Project says estimates within the industry put the added cost to consumers at around a penny per unit (note: I do not have supplemental evidence to support this claim). Here's a narrative  from the Enough Project showing how tantalum goes from these areas to your iPhone.
WIth the release of the iPhone 4, likely the biggest or second-biggest (after the iPad) gadget launch of the year, the Enough Project is mobilizing. They've organized protests outside Apple's new store in Washington, they've gotten an op-ed in the New York Times  from columnist Nicholas Kristof, and they've gotten a few actors to create an "I'm a Mac/I'm a PC" parody  (starring Joshua Malina, who I am forever unable to see as anybody but Will Bailey from The West Wing).
It's certainly a worthy cause, though I doubt tactics like protesting an Apple Store have much impact (besides, perhaps, press attention, which is probably what they're after). You can check out more about the conflict at the Enough Project's site.