Sorry, I thought you meant play in the lens cap itself when mounted. Should have re-read.
I don't have any play at all between the mount and lens on my 50 1.8 Mk II on a 20D body. Slop in the mount of any lens has always bugged me. Since you didn't buy it locally a replacement seems out of the question considering the cost, but maybe you should take a second look at it? Holding the lens in hand while on the body you can actually feel movement at the mount itself? What body are you using?
No slop in the mount, just a bit of play in the front element. I'm guessing that's just the way it is, not being USM and having a very light body etc.
Sorry, should've said, it's on a 350D

)
. It takes very very crisp sharp pictures, so I'm not complaining 
