Just installed
I installed an MX850 at home for Macs and Vista PCs to use last night. I have it on Ethernet and got printing working with minor difficulties. Just a few glitches with idiosyncratic software and Jinglish documentation on both platforms. You have to connect via a USB cable (not supplied) to configure Ethernet connectivity.
There should be a scanning driver included. It's in the marketing collateral, the installation guide and the printer comes with a scanning utility. I'm thinking that the latest Mac OS rev has caused difficulties with the Canon printing subsystem on the Mac. No driver updates on our local Canon site, but that's not unusual.
It's probable that push-scanning to workstations from the MX850 isn't supported for Mac. The doc implies there's a virtual TWAIN interface over the network from the Mac to the MX850. That's a shame because it's a really nice feature on the device. You can select the PC you want the scanned document to go to and it is transferred to a folder on the workstation.
That said, the MX850 was the only product that fit my criteria especially for PC and Mac support, colour, duplex printing and ADF scanning (duplex functions are slow, but they work), CD/DVD printing, faxing, fax/tel line sharing, separate ink cartridges and so on. HP doesn't have anything similar right now.
After a few tests, the printing seems good from both systems. My only other complaint is the software installation is a branding exercise for Canon with too many applications and their aliases installed everywhere.
Dekka