I have use this .Xmodmap in Ubuntu 11.04 and it works a treat for me.
keycode 66 = Shift_L
add Shift = Shift_L
I have just installed Linux Mint 13 and I put the file in my home directory ~/. Linux Mint either will not read it or something else is resetting the keyboard mapping. I have tried the GUI but the most I can do there is to disable the Caps Lock where I would prefer it to become another shift key.
Has anyone got any ideas what is going on and now I can work around this.