Those who missed the SpringONE 2007 event in Brussels can now view the presentation I gave on scaling-out of stateful applications entitled "Scalable As Google, Simple As Spring". It introduces the concept of SLA-driven OSGi-based containers, which are a lightweight alternative to traditional application servers. It is particularly geared towards fail-over and scaling of Spring-based applications, without dependency on J2EE.
You can find the online presentation here.
There is an interesting discussion on the Spring forum, End of App Servers?, which highlights some of the points I was trying to make in this presentation:
"Given the availability of all the various commodity services which an app server typically provides (connection pools, emailing, security, transactions, etc), there doesn't seem like much value-add provided by a war/ear/rar centric app servers any more.
Being able to just start up another OSGi container on another box, and dynamically register OSGi components to it seems much more attractive, in terms of manageability, scale-out/fault-tolerance."
If your interested in playing with it - i'll recommend starting by looking at the following webcasts