I was looking for info on the program that has the Harlem Line sim (couldn't remember the name of it) because I thought that was pretty cool.
At the same time, I came across this thread on MSTS. After seeing so much time pass since the latest post, I was surprised to see the "original announcment" on Microsoft's site appears to be unchanged, more than 9 months later.
From my experience as the webmaster of a fairly popular "SimCity" website in a previous life, I can tell you EA's policy used to be (and probably still is) that a game release date cannot be announced until they're 100% sure the game will be on store shelves on the advertised date. My contact there at the time said it had something to do with stockholders. Sounds crazy, but it made sense, especially since that wasn't the rule when Maxis was a privately-owned company before EA gobbled them up. I can remember several times, even younger, when highly-anticipated Nintendo games would be slated for a particular date, only to learn there would be a delay, then another one, and so on.
Long story short: I'm betting Microsoft will not announce a release date until the game is finished, the box art and manuals are finished and (most of) the bugs are worked out. At any point before that, announcing a release date could prove to be a bad move if, for example, someone discovers a critical mistake at the last minute, which requires an extra month to fix the code and reburn the CDs, or reprint manuals, or whatever else.
And yes, games are usually promised for a certain "broad" release date early on... like "4th Qtr 2007" or "January 2008," and sometimes these loose estimates are pushed back. But you'll never see something as specific as "September 26, 2007" until they're absolutely sure it's finished and copies can be produced and shipped in time for stores to sell it.