Steam Early Access is a relatively new idea from Valve, allowing players to help out in the development process by purchasing certain games early for the right to play them in their current state. The implied idea is that these games will eventually arrive at a final release. Now Valve is saying that might not necessarily be the case.
Valve has added a significant caveat to its Steam Early Access FAQ (via Rock Paper Shotgun), stating, "Its up to the developer to determine when they are ready to 'release'. Some developers have a concrete deadline in mind, while others will get a better sense as the development of the game progresses. You should be aware that some teams will be unable to 'finish' their game. So you should only buy an Early Access game if you are excited about playing it in its current state."
While Steam Early Access has always been somewhat of a "buyer beware" proposition, this is the first time that Valve has explicitly made it clear that developers may not finish their product in a timely manner, or even at all.