I just discovered I have signed up for more than one Google API. Is this against the rules and if so, what should I do? Also, what exactly are the APIs intended to be useful for checking?
Yes, this is against their rules.. You can create unlimited applications using the API, refer to their reference for more information: http://code.google.com/apis/soapsearch/reference.html#2_2 FYI, Google API does not return PR..
I think its 1 API per site. When you enter your domain name .. I think you will always get the same API key .. and if you try to use a different one the API will not work.
I have more than one site with an API. It's not a problem. In fact, you have to have a different API key for different domain names. You can't use two API's at one site.