Interesting article at The Grist under the Ask Umbra banner. Of most interest to me was the bottom entry on the sustainability metrics of e-readers. Short story even shorter? E-readers are a break-even measure once you've read around 100 books... Big reader? Then it's a good idea.
Unless you're like me and will use the e-reader (that I don't have yet) to read everything and then also buy the hardcover or TPV of the ones you REALLY, REALLY HAVE TO HAVE FOR THE LIBRARY WALL!!!
When I get one, I figure my break-even will be around 125-150...
The other good news is that paper books, hard or soft cover, are actually not so bad for the environment, even trees, as you might think, although the industry is still actively evolving (perhaps not as quickly as we'd like) to address it. Book (paper and digital) = good (but can be better!).