I recently invested in some FiveFinger shoes which aim to mimic walking barefoot.
They are very comfortable and I have done 3 x 20 min runs on grass with no problems whatsoever...after my first run the residual tightness in my left hamstring I have had for a couple of weeks went away completely!!!
The shoes are lycra uppers with a thin rubber sole a bit like rock climbing shoes and allow the muscles of the feet to move as nature intended as they are not restricted as in a normal shoe.
They take a bit of getting used to and the first day my feet were quite sore but I quickly got used to them.
Visit www.fivefingers.co.uk for more info.