Mount the lens on the body, lay the camera down on it's back and grip with two fingers the front of the lens barrel, then pull up the whole set carefully whit-out lifting it and try to move it sidewards (not turning!). If the lens barrel has a very little play then it is normal. But when you feel/hear it knock against the helicoidal, then there is to much play and send it back for a check up.:ugh:
I had the same play on a DA 21mm and this is what the Pentax Europe Service centre in Germany told me to do before sending it back, I had to send it back...
There must be some play in the lens system, because the AF need to be able to turn with the least of friction possible. Sometimes you can feel that the lens has less play when the AF screw is under 'tension' while focusing.
I hope you understood what I was trying to say...