###GolfScript, 120 characters###

{10{rand}:R~!{[{.}{;}{Z\?[.(.10-@).10+]{Z=}%' '-.,R=}]'QWERTYUIOP ASDFGHJKL  ZXCVBNM'' '22*11/*.{32|}%+:Z 2$?0>2+R=~}*}%

The code can be tested here.

{                      # loop over all characters
  10{rand}:R~!         # take random number [0..9] and negate (i.e. 10% chance of true)
  {                    # {...}* is the if-block
    [                  # Three possible operations (code blocks) in the arry
      {.}              # a) duplicate
      {;}              # b) delete
      {                # c) shift
        Z              #      Z is the keyboard layout (see below)
        \?             #      Find the index of the current letter
        [.(.10-@).10+] #      Calculate index of letter left, right, above, below
        {Z=}%          #      Extract the corresponding letters for indices
        ' '-           #      Remove any spaces
        .,R=           #      Take random item
      }
    ]
                       # Z is the keyboard layout (upper and lower case)
                       # with enough spaces around
    'QWERTYUIOP ASDFGHJKL  ZXCVBNM'' '22*11/*.{32|}%+:Z
    2$?0>              # Is the current letter contained in Z and not a space?
    2+                 # Add 2 (i.e. 3 for letters, 2 for any other char)
    R=                 # Take a random code block from above
    ~                  # Execute the block
  }*
}%