- top-down (business logic development): do as Kurt suggests and develop your business logic using whatever tools and methodologies you think are appropriate. From a high level perspective, this is as critical to your success as ever.
- bottom-up (service development): do as I suggest and think about the contract you want your service to expose and how that would map into message exchanges (content, sync versus async etc.); try to keep it as agnostic to the backend business logic implementation as possible (e.g., don't go exposing individual objects on to the bus). The service is your route into SOA: get it right and you'll be able to leverage all of the benefits the industry keeps pushing.
You can develop these two aspects sequentially or concurrently (even by different teams, which allows each team to concentrate on the right skill sets needed for each role). This is an aspect of what's traditionally called the service lifecycle part of governance. It isn't easy to get it right first time without appropriate tools and patterns. Plus experience helps a lot here: even with tools/patterns, you'll find it easier to do the more you do it.