Yer ive heard of the issue with the 40d shutter button before and it turned out to be dirt under or within the button compartment.
Canon 40d shutter button repair.
Don t waste your money.
This replacement is moderately to highly involved and should be allotted at least one hour.
As i said i would do a search about it 20ds seem to have this problem.
My 40d shutter failed after 16 700 actuations.
Originally posted ages ago.
Necessary tools for this replacement include a philips j000 screwdriver a plastic opening tool and a pair of precision tweezers.