It isn't quite the same as a physical book, but all the popular DRM schemes allow some sharing. The Amazon Kindle allows up to 6 devices on the same account. Adobe Digital Editions (most EInk Readers, except Kindle) allows 6 PCs and 6 portable devices on the same AdobeID. MobiPocket allows 4 devices per ebook file. The eReader format (B&N Nook, jetBook, iLiad DR800SG) allows as many devices as you want, but the password is your name and credit card number.
So if you are willing to share accounts, sharing of ebooks is certainly possible.