Definitely shutter failure, get it replaced soon before the shutter blade busts out and scratch your low-pass filter, or you will be looking for a $1000 low-pass filter replacement job + shutter replacement job
Think about where and how to have it replaced. I got the same problem on my 1Ds III as and when it returned from the Canon service center in California
after a general check-up and cleaning that I requested and paid for. It took three weeks to get the camera back and cost me $200 including shipping.
I then visited an authorised repair service in Europe which replaced the shutter free of charge for a new one. In their words, the shutter failure was due to a design/manufacture fault and Canon is supposed to have placed a recall on all 1Ds IIIs produced before a certain date.
No idea why I had not heard of the recall, nor why Canon California has not, but make sure you ask for a free replacement.