Valve has added a refund policy to Steam. They say that they do not care about the reason, but there are obviously restrictions including a blanket “abuse” clause that lawyers love. First and foremost, the refund must be done within fourteen days of purchase or two hours of game time. If you feel that your circumstance is an edge-case, even if you are outside these windows, you are free to ask for a refund anyway and Valve will take a look at it. Pre-purchasing is not considered a sale until the game launches, but “Early Access” has not been addressed. I assume Valve would handle that on a case-by-case basis. Valve says that refunds will be processed within a week.
This system is very similar to EA/Origin's refund policy, with a few obvious differences. First, EA's policy only considers “participating third parties”, although they fully put their money where their mouth is with their own catalog. EA's policy lasts seven days, while Valve's last fourteen. On the other hand, EA allows returns within the first 24 hours of launch, while Valve counts the first two hours of execution, seemingly regardless of how long that takes to happen.
We're hearing a bit of concerns from developers, especially those who create quick experiences. That's a bit of a hot-button issue, but I feel as though it is something that you will need to agree to in order to ship on Steam. Honestly, I expect that users will overwhelmingly not request a refund unless they feel slighted, even for a short game. It's a pretty convoluted way to pirate a game, for a brief time, and runs the risk of Valve cutting off the account from refund requests under the “Abuse” clause.
A final note: Valve will officially support refunds for titles purchased just before a sale. If you buy a game, and it goes on sale within the refund window, you can return it and re-purchase it at the sale price.