In reply to PaulC:
Google search for Inka, Fernando Grajales and Aventuras Patagonicas. They are three of the main in-country providers, the first two are local and many western or other operators use their logistics and facilities on the hill.
There are mixed reports on Inka, but there are mixed reports on most expedition operators in most places. I've done Aconcagua twice by the normal route, just using local companies (Grajales) for mules into Plaza de Mulas and overnight at Confluencia. Above PdM I've been independent, but from what I've seen of other 'guides' on the mountain it's not great. I'm sure some are good, but many aren't. You don't always get what you pay for. Caveat Emptor.
The normal route on Aco is very easy technically, but it gets quite cold and windy and it just a tad too high to be stuck at the whim of a glorified porter who'd rather be back at BC with his mates.
Have a look over at Summitpost.com for more info and reports from Aconcagua, including on operators. e.g..
http://www.summitpost.org/phpBB3/aconcagua-gear-help-t67726.htmlPost edited at 03:30