Buying DRM protected ebooks is fine as long as you maintain a platform to read them on. If you want to change to for example an eInk device such as for example the Sony Reader, you can wave goodbye to all your eReader (and other DRM protected) books. Even if an eInk device should support eReader books, you are limited in your choice of reader. (If you have no idea of what an eInk reader is, have a look at mobileread.com.)
This is a good argument for going for non-DRM protected books such as the multiformat ebooks from Fictionwise or from authors who like stevejordanbooks.com sell their books without DRM. Or make your own from public domain sources: I have made many great ebooks from Project Gutenberg texts.
I am still happy with reading on my Palm and PPCs, but when the eInk devices get cheap, I will want to change to one of these. And when I do, my eyes will undoubtedly hurt if I try to read on the comparatively horrible PPC screen.
