Two Reddit communities, Kotaku in Action and PC Master Race, have made note of bots on steam. You can spot them by looking at their profiles, many of which have only played games for approximately three hours, have weak names, and non-specific descriptions of the games.
This isn't that much of a problem as the bots don't appear capable of promoting themselves to be the top reviews. As such, if you scroll down you'll see human-given reviews with lengthy responses, and get a good view of what the game's like there.
Just watch out for 'Overwhelmingly Positive' indie games on Steam. Make sure you look at the reviews, watch out for anything fishy.
The mentioned communities quickly got to emailing Valve about the situation, who responded fairly quickly. The reviews were removed promptly. Three games which had these bots acting for them were ZombieRush, Base Squad 49, and Lands of Devestation. If you check out their Steam pages, you'll notice Valve's efficiency at work.
If you're curious to see just how bad it was, don't worry! Here's an archived form of the pages that shows all the positive bot reviews.
Hopefully Valve can figure out some way to prevent this from happening again. While there's little worry of big name companies ruining their reputations to do something like this, keep an eye out for indies desperate to make a buck by using underhanded methods.