It seems as if there are various questions, and "why not this..." comments going around in this thread. It also seems as if the Logitech mouse is preferred over the Microsoft or Belkin.
As to why it should have Bluetooth:
Anyone with a new Aluminum PowerBook isn't going to want to have to carry around another device to plug into their computer, just to use a wireless mouse. Bluetooth is built in, so there's no need for any extra receiving devices.
As to why not Microsoft:
Some people may find the Microsoft mouse ok, but the Logitech has a superior tracking engine. It is more accurate and smoother than the Microsoft one (from what I've heard). I don't think everyone here has the "I hate m$ so I won't buy their mouse" mentality.
So, if you're looking at the Logitech MX900, then you are obviously wondering "Does this work with my Mac". I don't see why Logitech hasn't tested it yet. They just have to take the mouse into an Apple Store, see if it works with a PowerBook, and then tell us yes or no.
I personally am looking at the Logitech MX900. If it works with the internal bluetooth, I'm not going to use the Bluetooth Hub that comes with the mouse. I'll just use rechargeable batteries, and charge them separately.
So if anyone here gives the MX900 a stab, and buys it to see if it works with the built in Bluetooth in the new PowerBooks, let us know how it works...