When I tried to renew my credit card details on Google Checkout today, I used my iPad - I am on holiday and don't have access to a PC. On my iPad, however, Google Checout only offers the mobile interface which doesn't have the pages needed to modify account/credit card details - nor does it offer a link to the Desktop interface.
Solution: download Atomic Web Browser, paid version ($0.99) > tap settings icon > select "Identify Browser As" > select Safari Desktop. Use this browser to access Google Checkout. Problem solved!
What this does is change the "User Agent String" which tells the server what browser you are using. I changed it to tell Google I am using Safari on a desktop rather than the mobile version of Safari on an iPad - so Google served me the full Desktop version of the website, with all the controls I needed to change my credit card details.
Note that while this worked in this case, it might not be appropriate for all uses - sometimes a website might require resources that simply aren't feasible on an iPad. An example would be a site with Flash content, or ActiveX scripts etc.