Here I documented my own efforts at modifying an 18-55 IS to fit the EF mount; I failed because I couldn't figure out how to sand the lens down to fit the mount without destroying the IS sensors.
Having converted the EF-S 18-55 II to EF mount, I decided to try with my new 18-55 IS.
Result? The EF-S 18-55 IS can't be converted to the EF mount. Vibration? sensors are housed in the EF-S mount, precluding an easy conversion that preserves proper IS functionality.
( from http://photography-on-the.net ...p?p=10183063&postcount=17 )
but the story gets better! I found someone on FM who had done what I believed to be impossible...
However, it does work at 19mm or so (the lens focal length encoder seems to not have a 19mm setting - it'll display 18 or 20) - the mirror barely hits the lens at 18mm. And while the corners aren't soft, they aren't exactly pretty either - they have extreme distortion.
This picture is the first -- and last -- picture my 1D3 will ever take at 18mm (w/ the lens zoom stop at the widest end)