More than 15,000 people are attending the Dreamforce conference in San Francisco starting Tuesday night-good proof that Salesforce.com is fulfilling its destiny as a cloud computing platform provider. (This many people wouldn't show up for a CRM conference.) Meanwhile, down in L.A., Microsoft execs are talking to developers about building apps to run on Windows Azure. Is that the crash-boom-bang of competitive thunderclouds I hear on the horizon?At Dreamforce, hundreds of companies, many of them startups, will be hawking their software-as-a-service apps, all developed using Salesforce.com technologies, running on the Force.com platform, and hosted in Salesforce.com data centers. Many of Salesforce.com's CRM customers attending Dreamforce will be evaluating these software services as extensions to their CRM subscriptions. All proof of the solid momentum behind Force.com.

Coincidentally (or is it?), Microsoft is explaining the technical ins-and-outs of Azure, which enters beta this week, at its Professional Developers Conference. It's still a bit early to see how Microsoft plans to market and make money off of Azure. But the way things are shaping up, it could become a direct competitor to Force.com.

One thing is certain: Salesforce.com is keeping a close eye on Azure developments. On Friday, the company's public relations team sent me some prepared statements about Azure attributed to Bruce Francis, Salesforce's chief strategy officer. It's the typical kind of stuff you see when one company wants to prove to the world that it's not threatened by another:

"When the company that has the most to lose from 'The End of Software' says that it's time to move Windows to the Cloud, you know that 'The End of Software' is near. The Force.com cloud platform powers more than 125,000 custom apps for customers today, and we believe that Microsoft's move will convince even more companies to consider enterprise cloud computing."

Now, even if Microsoft pursues a Force.com-type model with Azure, it's a few years behind Salesforce.com's momentum. Still, one sign of its potential is the willingness of a longtime Salesforce.com partner to jump in early with Azure.

Cast Iron Systems announced Tuesday that it is "teaming with Microsoft" to offer preconfigured data migration and integration templates for "moving hundreds of enterprise applications to Azure."

Cast Iron, a SaaS integration startup, owes much of its success to its Salesforce partnership, and it has a significant presence at Dreamforce. Quite a few Salesforce.com customers use Cast Iron's integration service to synchronize sales data, such as leads and forecasts, with ERP and custom applications. Cast Iron has some big customers, such as Allianz and Amerisource Bergen, and consulting company THINKstrategies today recognized Cast Iron with a "Best Of SaaS Showplace Award."

Cast Iron's announcement that it's "teaming with Microsoft" is no insult to its relationship with Salesforce.com, but it does show that it doesn't underestimate the potential significance of Azure.

As Salesforce's Francis points out, Windows Azure helps validate Force.com, and the general idea of building apps on the cloud. But at some point in the future developers will be making decisions about which cloud computing application platform should take top priority. A few years from now, they might be choosing between Force.com and Azure.