I took my 40D and 17-85mm (a long time ago) into the Sahara, I wasn't overly careful and I got no dust into the camera itself but it did creep into the shutter button and made the zoom ring on the 17-85mm grind (unfortunately with any lens this will happen, even weather sealed Ls)
A few months later I was shooting in the Lake District in Cumbria England and it pored down and with the rain the dust from the desert was pushed further into the button by the rain and the button stopped working. That was it for the camera it was pretty much a gonner.
Only fix was to pore alcohol down the battery compartment and it come out of the button. I had already bought a new camera and the cost to fix was more than the cameras value so it was last resort, but it fixed it and it continued to shoot for another day.
You should be ok with the 7D its weather sealing is better but dust/sand with the high winds in the desert and at the coast gets into everything.
Ben Long has just done another practising photographer video series on Lynda and goes through all of the above, his 16-35mm and 24-105mm got the grind in the zoom ring its just something that happens with use and was quoted $400 to fix... But for me the pics are worth more than the gear so its a personal choice.http://www.lynda.com/Photography-Cameras-Gear-tutorials/Travel-Photography-Desert-Road-Trip/121831-2.html