This article from the Washington Post highlights a problem which is bound to increase in frequency. It discusses the fact that Walmart has just cancelled their digital movie download service.
When you lock yourself into some form of entertainment which makes use of DRM, you're at the mercy of the company providing the data/service to continue offering it. Now the people who have downloaded movies can only continue watching them on the same PC they downloaded the file to in the first place and they can't re-download should their PC hard disk fail. I'll concede it was more convenient to purchase a movie this way but there wasn't a significant savings involved and what do they have now? Data subject to rapidly increasing entropy thanks to shifting business priorities. They'll definitely lose access at some point... it's just a question of when.
I think the same problem will occur with the e-book readers currently on the market. The Kindle (from Amazon) certainly makes purchasing and carrying lots of books much more convenient and even gives a pretty good ease of use, something important in a device designed to replace physical books. But at what cost? The books themselves aren't offered at a huge savings over paperback editions and they're subject to DRM. So you're tied to the device and should Amazon decide that the device isn't profitable enough and discontinue the service, you're stuck with an expensive device which will continue reading books you've already purchased but won't be good for much else. If they discontinue the service, you can bet that the wireless connection will no longer work.
I've also got other concerns about the device. Can you back up your purchases onto a PC or are you forced to re-download them if you want to re-read them in the future? There are some books I like to read again periodically so a proper replacement for physical books would have to take this into account.
Some companies have little interest in guaranteeing customers have continuing use of their DRM laden files. Microsoft changed DRM formats when they introduced the Zune. It won't play any of their previously purchased songs using the earlier Microsoft DRM technology.
The real answer is to avoid all forms of DRM. Applaud the move of 3 of the 4 major record labels to offer DRM free music on Amazon by supporting their efforts. Whenever possible, support the work of independent artists who haven't been punishing their customers the way the RIAA has. But most importantly, put your foot down and say hell no to DRM.