Did you notice that visiting your Google account through AdWords, AdSense, Analytics and other SSL-required sites is giving you a security warning? Search Engine Land did. And that means that Google has forgotten to renew its SSL certificate. Apparently this is only a lapse in licensing specifically for the googleadservices.com certificate, and not the one needed for google.com.
And Google isn't the only one not using iCal reminders. Search Engine Land goes on to point out that Yahoo has a similar mishap earlier this year.
Given the magnitude of Google, however, are you likely to be effected by this? You may still be comfortable with it on a personal level, but as a business with Google Checkout as an option on your site, visitors may be less understanding of your plight, and will blame it on you. Now, you could make a big ado about the whole situation and demand some reparation from Google, which will likely amount to 25 cents or less, which will be applied as a credit to your Google Checkout account.