Online gameplay with Windows machines will most certainly be available. We've been able to interoperate with Halo -among various other games- for years!
I'm very excited about the Need for Speed series coming over to the Mac side. Before they went the "Underground" route, the games were very good. I loved "Porsche Unleashed" when it came out, and still play it every once in a while on my archaic PlayStation 1.
Also, I'm not exactly complaining about Harry Potter coming over; looks like it's going to be very interesting to play.