Ah, I think the unstablity is for the Lithium-ion based batteries, not Lithium based in general, sorry

But Li-Ion is mostly used in laptops, mobiles and any devices that would draw high current at any time. I read that they are dangerous if safety features are not implemented. If the positive and negative terminals are shorted with some accumlation of microscopic metal particles (that may have come during the manufacturing processes), there might result a sudden explosion, or "thermal runaway". There are alot of other factors too like thermal unstablity above a certain temperature.. Anyway, Im not too familiar with the technical details
If you are wondering WHY we have this technology if its dangerous in the first place, its because Lithium-Ion based cells provide many advantages over previous technologies, like it can recharged a number of times, has a high energy density and is environmentally safe (good for disposing). I suppose manufaturers do try to find ways to make it safer.. but theres a limit on how well they can do this, plus they still have to consider the cost factor too.
Here's a site with a lot of useful info:
Lithium-ion safety concerns
http://batteryuniversity.com/partone-5B.htm