Interesting thing you've pointed out there. And thanks for writing, elaborate like always it's good to read.
Been trying to connect to a salesforce account with this zimlet. This one is not a trial account so I am assuming he is having a enterprise subscription. But I always end up with this error 'API_DISABLED_FOR_ORG:API is not enabled for this organisation'. Please correct if I am getting this wrong, I believe the customer has to purchase this enterprise account that has this API to connect from apps like zimbra (understand your point that its free with gmail)
I believe his account is such API enabled and no clue on what is denying it. Any help would be much appreciated. Good luck.
PS. using the latest 2.0 salesforce zimlet. Anyway to check if the account is API enabled? perhaps from the salesforce site.