Right now, a digital e-book reader (like the Amazon Kindle or the Sony Digital Reader) has a very focused purpose: books
In the near future, I believe they will evolve dramatically. They will incorporate color screens, have Wi-Fi (some already have this), a web browser, multimedia capabilities, e-mail, and much more. Wait, did I just describe a tablet PC?
As the prices of mobile computers continue to drop, it will only make sense to make these digital e-readers more powerful. They don't need to have all the capabilities of a full-featured tablet PC, but they will have enough so that you can get online, get some work done, and use it for entertainment.
My prediction is that the upcoming Apple tablet will be a device that will catapult the e-book reader audience to fall in love with additional tablet PC features. Eventually, these devices may be called "net-tablets" similar to the way we call mini notebooks a netbook. At some point, the line between a digital e-book reader and a tablet PC will get very blurry and we'll see so many overlapping features that we won't see dedicated readers that have a very focused purpose.