The Google Checkout Blog announced that they now support Google Checkout over mobile devices and mobile enabled sites.
The only difference, for a buyer, is when checking out over Google Checkout mobile is that they verify themselves with a PIN as opposed to their Google account login (i.e. email and password).
Merchants who sign up today will automatically have the mobile functionality, however old merchants need to sign up for the mobile functionality – it is free. More details at http://checkout.google.com/support/bin/topic.py?topic=11659.