There may still be IT executives who scoff at cloud computing as something for small firms and bleeding-edge pioneers, but don't count Andrew Brown among them. Managing director, head of strategy, architecture and optimization at Bank of America, Brown declared last week that blending internal and external cloud capabilities will be the challenge for enterprise architects over the next five years.

"You have to come up with the right blend of where you use the cloud and what you keep inside," Brown said at a recent Saleforce.com New York preview of its Chatter service. "The most important part is how you join them together."

Interviewed by Salesforce.com CEO Marc Benioff before more than 1,300 people packed into a mid-Manhattan hotel ballroom, Brown laid out fundamental questions he said enterprise architects must answer over the next few years.

"I think most forward-looking companies are looking at internal cloud solutions, external private cloud and external public cloud," he said. "The question is, how much data do you feel comfortable moving out and what level of service-level agreement can you obtain from internal, external private and external public cloud offerings?"

The choices enterprises make, Brown said, will depend on the SLAs, prices and data security guarantees available at each level. What will it take to synthesize all three environments? "The winners will be able to create a near-real-time services architecture to go with the human [collaboration] architecture," Brown said, noting that architectures will have to securely expose events inside a company to the outside world.

Chatter is a free collaboration service Salesforce plans to add to its sales and service applications and cloud platform sometime this summer. Nearly 50,000 of Bank of America's 300,000 employees use Salesforce.com's CRM application, and Brown said the bank's beta tests of Chatter have been a success. The service will embed social-networking-style collaboration alongside existing Salesforce.com functionality and it will integrate with popular public networks including Facebook and Twitter.

"As a customer service company, whatever channels our customers are using, we need to figure out a way to be present there," Brown said. "Chatter lets us collaborate in real-time with our customers, and we're able to close a number of [customer service] issues there."

As a top IT executive at one of the world's largest companies (and a security minded bank at that), Brown's assessment might just change a few minds about how long we'll have to wait for mainstream enterprise cloud computing.