It's good of Valve to do this. With Steam being such a huge provider, it's likely that other developers/publishers will look seriously at doing native Mac ports of more games.

Valve's advertising cracks me up.  "I hate normal." heh heh.

Right now, you'll still see greater sales (usually) in titles that are released with both DL and hard copy format, just because many people are still more comfortable with retail buying. Plus it's just a wider avenue for sales - like having an anticipated TV show premiere on two nights instead of just one.

But we'll see that changing. The typical Steam contract is 25/75 - Valve takes 25% of sales off the top and the rest goes to the publisher, who shares with the developer as it sees fit.

A typical retail contract between developer and publisher is 12/88 - the publisher takes 88% of sales until costs have been paid, then sometimes (not always) increases the percentage that the developer receives. In certain cases, the developer will receive special bonuses for performance - an extra X% for a million copies sold, or what have you.

The thing is, Steam is a distribution platform, not a publisher. Publishers do more than just packaging and marketing; they fund the development of the game. It costs $10M+ (sometimes a LOT +) to make a top-shelf game these days, and the dev cycle can be between 18 and 36 months. Few developers have that kind of money lying around, so the publisher typically ponies up 85-100% of development costs, paying out across milestones. As such the publisher is assuming enormous risk should the game fail or be delayed. That's also why they take 88% off the top: they need to recoup their investment. In most instances no one sees any actual money until ROI is achieved.

Steam and other channels will change things enormously. It's made it possible for very small indie developers to make massively successful titles (World of Goo, for example); and since 2D Boy didn't have a publisher, it took the 75% of Steam sales. And Steam's wide reach exposed millions of players to a game that'd never have gotten traditional shelf space and that many would not otherwise have heard of.

Another thing to keep in mind with Steam is that Valve doesn't do things half-assed. Those Steam holiday sales? Very cunning. Often you'll see games that were $40-$60 marked down by 95% or more. Seems crazy, right? Yes, until you consider the data Valve's released indicating that sales of any given SKU increase by around 18,000% if the price is cut 75% or more. After all, who can resist Bioshock for $1.99? They and the dev/pubs see hella profits from those sales, and since Steam is always rotating sales, someone is always making out like a bandit.

Steam sales numbers refer to revenue, not unit. Thus when they released Psychonauts for $2 and it shot up to the #2 bestselling position on Steam, it's not that it was dominating in unit sales. It was the second highest-grossing game that week, despite being only two bucks. So based just on that, you can see the kind of sales power platforms like Steam wield. 

And of course it's only a matter of time before Steam starts funding development. There's just too much money in it for them to stay out of that game. The only question is whether it will stay part of Valve or whether the company will spin it off.

Steam probably could charge a lot less than boxed and still make a profit, but retailers typically bind all delivery mechanisms into contracts so the prices are the same at launch.