I have shot in the Sahara in the past, and not suffered any particular sand/dust related issues arising from simply shooting in a normal fashion. So long as you have a front filter on, I don't think you need to degrade the image with a plastic bag on the front. But do be prepared for a coating of dust on the outside of any kit, and I recommend careful wipe down of camera/lens before detaching the lens.
What I can strongly advise against is changing lenses in 'the field' , no matter how careful you think you are - big mistake I made on my first trip. Also, valuable tip I received was to take your camera bag inside a plastic bag - depends on size of bag of course as to how practical this is - I used Billingham bags, and whilst both handy and good looking (to my eye), they are certainly not dust proof in the desert. Never mind how you travel (windows up etc) the sand and dust eventually gets into anything not sealed !
On subsequent visits, I used large ziplock bags for each item, inside the camera bag, with the camera bag in a larger 'bin liner' type bag when travelling in vehicles (sat on back seat etc)..
I shoot mostly primes, so I can't comment if there are any additional dust issues related to zooms - but don't think the 17-40 should be much of an issue?
Enjoy the light - and just be prepared for a sensor clean when you get back!
Stoical.