In the future, biometric authentication will solve this problem. And your voiceprint solution is along these lines, of course. I have three suggestions, two of which do not solve the problem, but the third of which might be more promising. The first is to embed something customer-specific in the material at delivery time. For example, a prominent header stating "Training Provided for ". It's a weak deterrent, but it would discourage some people from sharing. The second is to require multiple pieces of information at logon time, and to have them be quite personal. Not just a password, but year of birth, place of birth, what car they drive, etc. Randomly demand two of these pieces of information at each logon attempt. This will not be inconvenient for the original client, but it will be for others. Again, though, it's not a perfect solution if people are determined to share. My third suggestion is a development of your third suggestion. At purchase time, generate a unique URL for the training. Shut down the URL after a short amount of time (say, three days, or even one day). But, rather than making the training material permanently inaccessible, email a new and valid URL to the client if they try to access the defunct URL. So if they share the material with others, it will time out. But if the original client tries to access the material, they get a replacement URL in their email and can continue to have access.

Mike, what type of permission does a blogger such as you need in order to link to someone's photo that is located (for example) in their Flickr stream? I presume that any photo tagged as "all rights reserved" is off limits, even for linking to -- or am I mistaken? Would some type of non-commercial creative commons licence do the trick? I'm sure many people would be pleased to see their photos linked to by your blog, but would not want their photos to be used commercially without permission.

The photo that is on the cover of the UK edition is currently on display at the San Francisco Museum of Modern Art, as part of their Munkácsi exhibition. But it is hung there at right angles to the way it's displayed on the book cover (woman's head is at top right). I think it's a more powerful composition the way it appears on the book cover. I wonder which way Munkácsi intended.