You want a system which can somehow detect the difference between:
- The user is on the chair in front of his computer, looking intently at the screen (for "reading") but not otherwise making his presence known to the computer.
- The user is on the chair in front of his computer, looking intently at the back of his eyelids, because he is taking a quick nap.
Said that way, it becomes rather obvious that the problem is hard. You could imagine plugging a Webcam to the computer, focused on the user's face and able to detect whether his eyes are shut or open... this seems kludgy. However, such a product is apparently on the market. You might want to give it a try -- not for automatic login (it is too easy to fool a face recognition system with a life-size photograph of the user), but for the automatic lock when the user leaves the room. Of course, this requires Webcams. Laptop computers have them.
The normal way to deal with unsolvable problems is to change the problem until it matches a solution. The common method, in this case, is to "empower" the users. This takes the form of a tradition of users jumping on each other's machines: if you find an unlocked workstation, then you send an email to the whole floor in the name of the workstation owner. The said user is then "forced" (through social pressure, not contractually -- it would probably be illegal) to pay for cookies or beers for everybody on the floor. If done properly, then the rate of unlocked workstations can significantly drop.
Unfortunately, this does not work well with people whose hierarchical level is higher than their sense of humour. The same people will also be uncooperative with any technological constraints. For instance, you could enforce smart card logon (with USB-token form factor, and automatic locking when the card is extracted) and have the users keep the smart card tethered to their wrist of belt at all times. It can be easily predicted that the users will not keep the USB token attached.
This issue is widespread. As an example, the French government just recently issued a reminder to its own ministers that they are not supposed to use "not approved" smartphones, and yet they do. Similarly, Barack Obama, upon becoming president, adamantly refused to let go of his phone, to the dismay of the Secret Service.
For people at the top management positions, the most practical method might be to simply hire an underling whose job is to type Win-L whenever his boss leaves the room. If that employee is decorative enough (which, in many cases, means that the employee is a she and is young) then such a plan will meet user approval, and be effective.