No, Ubuntu will not switch between the cards. This is known as a hybrid graphics system.
If you look in your
/var/log/Xorg.0.log you can see which driver is loaded (page down a few screenfuls and look for RADEON(0) or intel(0).
The only way I know of to force which card to use is to specify the PCI Bus ID of the graphics card. You get the Bus ID from lspci. It's also usually shown near the top of your
/var/log/Xorg.0.log. Then, set up your
xorg.conf with a device section and put
or whatever bus id you want. You don't need to specify the video driver as well (but it can't hurt).
David Airlie has worked on making hybrid graphics work better. His blog has some interesting info about it...
Searching for 'linux hybrid graphics' may turn up other useful examples and details about it.