In reply to vim_meister: I wore my Salomon Thermic Guides (masters of pain) on Aconcagua, but they would have been too much on the walk in. It gets really hot in the Horcones valley, so I was advised to send them on ahead to base camp by mule. Everyone on our group did the same and walked in wearing light approach shoes.
You have to cross the river in a couple of places, so if you are unhappy with the idea of bare feet and quite sharp stones, take a pair of Tevas or similar - not flip flops as the current is v. strong and one of our guys lost one.
Our expedition was self organised, so we had to sort out mule transport ourselves at Punta del Inca.