Big news, big potential
First of all, if Valve has truly ported Source over to run on Mac OS X then it has done a potentially great thing. Right off the bat, it means fans of these games that like Mac can actually play them without a separate machine or running Windows on their Mac.
Down the road, these Source games, which are extremely popular, will hopefully attract a largely untapped Mac gaming market. This builds momentum: as the market for Mac games grows (demand rises), other firms want to get in on the action. This means more companies seriously considering releasing and actually releasing their games on the Mac platform.
The lack of a variety of popular games on the Mac has has been a vicious cycle; game companies don't see a big market and don't develop Mac versions, Graphics hardware manufacturers who sell a significant portion of their hardware to gamers see the small market and don't make many Mac-compatible cards--this pushes up prices, high prices means gamers are priced out of the Mac market in favor of PCs and consoles, which continues to shrink the demand for games on the Mac, which continues to shrink the supply of games released for Mac, which continues to shrink the graphics cards available and raises prices, which ...
This could be a turning point in the cycle, providing current Mac users with some very popular games immediately, and provide incentive for other producers to offer content to the Mac via Steam.
I, for one, am very excited to see what games will be available for the Mac and where this will go!