Johan Strandler from InfoQ did a good job of describing the architecture implications of scaling stateful applications in his article New patterns and middleware architecture needed for true linear scalability?
Is there a need for new patterns and middleware-architectures when building linearly scalable applications? Nati Shalom, CTO at GigaSpaces, believes that existing middleware designed for a tier-based approach cannot be used for a true linearly scalable architecture. Instead he proposes a new middleware stack based on self-sufficient processing units that supports a partitioned/scale-out model. While Shalom proposes a new middleware stack, Pat Helland at Microsoft, some years ago proposed some new transactional patterns and formalizations to be used in what he calls almost-infinite scalable systems.
This is an encouraging sign that awareness around what's needed to achieve true scalability is now center stage.
