I've used my Canon MP640 for some time now and everything works fine. This is how you can get your MP640 working as well.

NB. If the driverpackages you download from Canon have different names, adjust the terminalcommands accordingly.

Installing the printer

1. Download the drivers from the website: http://software.canon-europe.com and extract the .tar file. Inside the extracted folder there is a .tar file called MP640_debian_driver_pack.tar. Also extract this file. The last file to extract is called cnijfilter-mp640series-3.20-1-i386-deb.tar.gz, located inside the previous extracted file.

2. To install the printerdriver, you have to open and modify install.sh, found in cnijfilter-mp640series-3.20-1-i386-deb. I've taken the instructions below from this site:

However if you try to install these drivers on 64 bit Ubuntu you get the architecture mismatch problem.

package architecture (i386) does not match system (amd64)

You can get around this problem by adding the --force-architecture to the dpkg command line in the install.sh script.
The dpkg line should appear like this in the script
sudo dpkg --force-architecture -iG $c_fpath_pkg_name
You also need to add it to the allowed arguments
&& [ $C_argment != "--force-architecture" ]

3. Make sure the printer is connected (either with a USB-cable or the wireless network).

4. Install the printerdriver by double-clicking install.sh and choosing 'execute in terminal'. You'll be guided through the printer-setup. See the manual included in the .tar file for an explation of the various steps.

5. When the setup is complete, the terminal will close and you're able to use the MP640 printer. Using it is pretty straightforward, but see manual from Canon (included in the .tar file) on how to use the printer if you're having difficulties.

NB. To check the printerstatus (inklevel, etc) you can use the 'cngpijmonmp640' command

Installing the scanner

1. Download the driver from the website: http://software.canon-europe.com and extract the .tar file. Inside the extracted folder there is a .tar file called MP640_Linux_Scangear.tar. Also extract this file. The last file to extract is called scangearmp-mp640series-1.40-1-i386-deb.tar.gz.

2. Open a terminal and navigate to the 'packages'-folder inside the extracted scangearmp-mp640series-1.40-1-i386. Run the following commands in your terminal: 

sudo dpkg --force-architecture -iG scangearmp-common_1.40-1_i386.deb

and

sudo dpkg --force-architecture -iG scangearmp-mp640series_1.40-1_i386.deb

3. Download getlibs from here and install it.

4. Lastly, run the following command inside the terminal:

sudo getlibs --release lucid /usr/bin/scangearmp

5. Your scanner is ready for use. Use the 'scangearmp' command or start up GIMP. See the manual from Canon on how to use the scanner.

Enjoy your Canon Pixma MP640!