The answer is, not very open.
Like many companies, Salesforce.Com is using the idea of open source as a marketing gimmick, while harnessing some of its practices to a proprietary business model. CEO Marc Benioff has a right to be proud of his success. (That's him, from the company Web site.)
There is nothing wrong with that, so long as we're all clear on what is really happening.
In this case, we have a hosted application server allowing customers to freely exchange code (even lease code) which supports Salesforce's service model. For any of this to be useful, you have to be paying Salesforce for its service, whether you're providing or getting the code.
Again, nothing wrong with it. You can say the same thing about Microsoft. Developers and those who buy from developers are all Microsoft customers. But is that open source? Of course not, but Microsoft has always considered itself to be offering an "open" standard, one that can be enhanced in many different directions. This was in contrast to the "closed" IBM mainframe standards that came before it.
Salesforce, in this sense, is equally open.
In the case of AppExchange, trials are free, but offerings 'from partners will typically require a fee,' according to the press release. Salesforce enhancements will be free of charge. AppExchange launched with 70 applications, the company said.
It is a great business model. It seems to work for customers. Salesforce is continuing to gain marketshare in the CRM space, and its Dreamforce conference in San Francisco has 3,000 happy participants.
Hosted application services on an enterprise level have long been a dream of the software industry, something Salesforce is making a reality.
But is any of this open source?
Not on your tintype.