Sony's FAQ at ebookstore(dot)sony(dot)com makes it sound rather simple:
Originally Posted by Sony
You can give an eBook to a friend if he has either a computer or a Reader Digital Book authorized to your account. If your friend has an authorized device, he/she will be able to download the same eBooks that you downloaded for yourself. However you can not share eBooks between 2 different accounts (email addresses).
Assuming that this statement can be interpreted literally, a device can only be authorized to one account at a time. Thus, in theory, I could 'lend' a friend a DRM'ed eBook by authorizing their Reader or PC to my account, and s/he would be forced to de-authorize his/her reader while reading 'my' book, but could re-authorize it to their own account when they finished reading. I have done a similar process with Audible(dot)com files. Can anybody who has bought from both Sony and Audible tell me if that's how it really works in practice?