How do you write a business case to justify your SOA project to your executives?
"Best practices for a quality SOA business case" (SearchWebServices.com) has some tips. The gist is to not focus on SOA, but to focus on the benefits the approach (which happens to be SOA) will have on the project. Explain to the executive what's in it for him/her, which is not SOA, but the business value and ROI of using SOA.
IT gets a lot of benefits from using SOA, such as greater reuse and faster development via reusable services. The #1 business benefit is business/it alignment. A book that explains the business benefits of SOA well is The New Language of Business.
Thanks to Bharath Duggirala for pointing out this article to me.