Take a "traditional" vertical application stack containing a web / middleware / message queue application and re-architect it for improved scalability
Anoniem
Split it out to scale horizontally (multiple load-balanced tiers) and outline how this is deployed via common devops tools (Puppet/PowerShell+DSC). Suggest AutoScaling if they ask how this can work on AWS. (SEEK are a small company, but naturally their apps are high-traffic - they really like folks who can demonstrate high-availability architecture knowledge.)