According to Diamond Club customer service you can get past security with a print of your itinerary with the bmi logo or on your travel agents headed paper.
Haven't actually used e-ticket yet, but have to say I don't fancy being turned back by security. Conversely I'd be concerned if they let me through with a simple printed A4 page. I'm sure I could create a forged bmi document within half an hour - not really the kind of security you expect.
I know they planned to intoduce e-ticketing and check-in machines last year, but think it was dropped as a cost saving measure. They need to be careful that they don't alienate business travellers by subjecting us to easyJet style check-in procedures.
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Nick