Unfortunately, that makes this just a rumor (it's certainly possible the icons were simply included in some stray bit of code, as they appear to be simple window controls, not actual game code), but if there's anything to this at all, it would be huge for us Mac gamers: Steam comprises a gigantic library of present and classic PC games, and bringing even a portion of them over to the Mac would be terrific. CrossOver Games has been working with Steam in the past, and they already have most of Steam's biggest games up and running on the Mac platform through emulation. If there's a partnership between the two companies, or if Steam is planning to take advantage of the demand for their games running on OS X, that would be great.
We'll keep our eyes open. Ever since Apple switched over to Intel, there's been more potential than ever for Mac gaming, and Steam on the platform would be amazing.