Here's how I got things working under 7 x64 Ultimate:
1. Pop in the installation CD. Select custom install.
2. Install the MP drivers, Canon IJ Network Tool, My Printer, ScanSoft OmniPage SE, and the MP Navigator software. If you want to install everything else, knock yourself out.
3. After that's done, reboot.
4. Go to the Canon MX850 website, and download My Printer v2 and the IJ Network driver.
5. Install the updated My Printer.
6. Extract the IJ Network driver, then go to the mx850sndwin250a_ntwin250aen\mx850sndwin250a_ntwin250aen\LANUTIL\English folder. Run the setup application. You should now have the network scan utility in your tray.
7. On your printer, press the scan button. Your computer should now be listed.
On Win 7 x86, I just installed directly from the disc, then I only downloaded the updated software (no drivers) from the Canon website. The scanner worked over the network perfectly without any need to update anything.
Hope that helps. I'm hoping Canon will release Windows 7 drivers in September or early October, before the official launch.