From the Google Terms of Service :
5.3. You agree not to access (or try to access) any of the Services by any means other than through the interface provided by Google , unless you are permitted to do so in a separate agreement with Google . You specifically agree not to access (or try to access) any of the Services using any automated means (including the use of scripts or web scanners) and must comply with the instructions set forth in any robots.txt file present in the Services.
(in italics)
More specifically, when you say “Google APIs,” which one shows the many APIs that Google provides , do you mean? Each of them has different conditions of use. Some may be used commercially, and some may not.
Update:
of terms for Picasa Button and Uploader APIs
7.1 In accordance with the Terms, you can develop, display and / or distribute your button using the Services as part of a commercial or non-commercial enterprise, and you can develop your button for use in accessing paid content or services. You may not be charged separately for using any button unless you have entered into a separate, signed agreement with Google. If you want to sell or transfer your button, you must obtain the written permission of Googles.
from terms for Picasa API Web Album Data
5.8 You may use the Picasa Web Albums API as part of a commercial or non-commercial enterprise in accordance with these Terms. However, you cannot charge a fee for using the Picasa Web Album API unless you have a separate, signed agreement with Google.
Regarding the Google Reader API , it still seems to be unreleased, and so no specific conditions exist.
To conclude a signed agreement with Google, it is probably best for you to contact your local Google office and speak with one of their business development representatives there ... Alternatively, you can try contacting the Business Offer department from your contact page .. .