What you need to bear in mind here is that the bulkheads can't be used for excessive storage, one of the learnings of the Kegworth tragedy. Moreover though you are allowed to hold handheld devices during taxi, take-off and landing, the rule of thumb there is 1 kgs, not that anyone is going to weigh anything here. But essentially that covers mobile telephones, Kindles and at a pinch smaller tablets.

In between you've got common sense, and generally speaking being hit at head height by a flying laptop as a result of the brakes coming on during the faster stages of taxiing (which feels fairly similar to hitting a brick wall at 20 mph in my one and only experience) could have really bad consequences.

So yes I guess there is a grey area. If your tablet / PCis smaller than Highlife magazine then I think you would be OK. If it is larger than Highlife magazine then for mainly your own safety it would be best to go in the overhead storage for a few minutes. I seem to recall there is another weight guideline for this too, but at the end of the day if the person securing the cabin is unhappy with a risk, it's the risk that gets sorted, not the person doing the job.