You can read a whole book on your BlackBerry (or your Palm Treo or your Windows Mobile phone) and you can enjoy doing so.
I know you don't believe me, so I'll say it again.
Since you carry your "smart phone" around with you all the time anyway, you might as well get the most out of it . . .
. . . I was surprised at how enjoyable I found the mobile-phone e-reading experience; you can get lost in what you're reading as much as you can with a printed page.