As Frank Sinatra often crooned, "Love and marriage, love and marriage, go together like a horse and carriage." It's pretty much the same relationship between services-oriented architecture (SOA) and software as a service (SaaS), and as the song would have it, "you can't have one without the other."
From a SaaS delivery perspective, SOA is what separates the current generation of SaaS providers, epitomized by the likes of Salesforce.com and NetSuite.com, from the failed application service providers (ASPs) of the dot-com era. From a consumption perspective, you certainly don't require a SOA to use SaaS, but if you want to effectively mix and match external services with on-premise assets and services, SOA will make it possible to efficiently build, deploy and manage composite apps.
