We have done a similar integration, that is using webservices as the backend for our seam beans (but we use plain axis 1.2 wrapped in spring)
While all of our seam beans are session/conversation scoped, our spring/webservice beans are stateless. Our seam beans call the spring beans. And not the other way round.
There should be no need from a spring bean to call a seam bean.
If there is really the need in one or two places, you could call Component.getInstance() manually (we have used this in an AOP aspect that wraps some of our spring beans for logging/authentication/error handling)
And yes, Seam in Action is a good book, including the chapter about all levels of seam/spring integration.