What MS should do with Halo 3 (and shoud have done with Halo 2) is release the PC version much sooner ie 6 months - that still allows them to capitalize on the 360 sales first.
A PC release of the game some 6 months afterwards would make more sense, if they want to catch the attention of the PC gaming crowd.
Usually, most games sell at their best straight off the "new release rack", as soon as its releases in the first few weeks and all.
But the thing is you can't even let people know you're planning on releasing it in the near future for the PC or else a lot of people will just wait for it. There's nothing immoral or unethical about the way they do this, it just makes sense for them to do so.
I held out for Jade Empire and Fable on the PC. C'mon, we all knew that w/ games being made by former PC companies (Bioware and Lionhead), so you knew it'd get a PC port....eventually.
And those two, they were done justice -- graphical upgrades, some extra content, tailor-made PC -style controls, etc etc.
But yeah, people will still care in 2 1/2 years or else you guys wouldn't be talking about Halo 2 right now. Beyond that, if you limit yourself to only one gaming platform, obviously you're going to have to either miss out or wait for a lot of games.
One of Halo 2 PC's biggest mistakes is being a Vista ONLY release. It'd probably make more sales, if it was coming to the Win XP and Vista, I'd bet. Not everyone's thrilled about Vista and wants to upgrade to it.....
And I don't think Halo PC's reputation is helping matters, w/ Halo 2 PC approaching. Most people knew this game for its technical issues (bad framerates), not-so-great PC graphics that never needed to support DX 9.0 Shaders in the first place, and repetitive "cut-n-paste" levels, and repetitive gameplay in the game's 2nd half....
And yes, wait I shall -- and even more so, for the games to hit the "Bargain Bin" rack and 'til I get Vista. That might be a pretty good while, from now.