Though I don't have a lot of experience with wireless scanning, I found that it can really be hit or miss as far as remote scanning goes if you don't have the manufactuer's software installed. For my Epson, for example, Windows 7 can *find* it as a scanner, but it often reported communication errors until the Epson network software was installed. It was even exasperated when I had it connected by USB as well. The network portion and the wired portion had at it for a while till they sorted out on a reboot.
On W7, open up the Devices and Printers section and see if the Cannon is showing up as a scanner. You might have to enable the network browsing service if it isn't on, but if you're not on a domain it should be good. If it is showing up, see if you can do any scan functions by initiating it from the software side vice the printer side.
I'd also say go ahead and see if Cannon has any software package for the printer to see if that makes a difference. Their Besides, Windows 7 default built-in scanning functions aren't all that feature rich, so some additional manufacturer's software might help (even if is is bloated...I'm looking at YOU HP). Also, check Cannon's troubleshooting KB for info. Search for "scan", then 3rd and 4th options down for the usual routine of troubleshooting wifi scanning.