Yes it will. Games on valve keep both their Mac and PC version in one copy.
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If that doesn't say enough about VALVe.. Well it doesn't actually. Nothing can tell you how amazing VALVe is. If you have not played Portal, do that now. Notice the company took their time to port this old game over to the Mac just so they can sell them for cheap. They care about customers, not money. Making games free for a limited time before they make a sequel... thats great marketing that makes everybody happy!
LOL, every and each company cares about money.