App Engine's Matcher, which went out in the 1.3.8 SDK, is a great example of where Platform as a Service is headed.
What can you do with it? Register ~100,000 queries and run them in parallel against ~1000+ new documents every second. This is the mechanism you need to build track at scale.
The queries it supports are complex (compounds, numerical ranges). The latency is low and not a function of total queries. Its efficiency and footprint are impressive. Building and deploying infrastructure that does this same task yourself is incredibly difficult. Some sites famously launched features in this problem domain only to shut them down due to scalability issues.
To me this represents another example of how cloud platforms have begun to exceed what you can do yourself with a big bag o'machines. This goes far beyond ease-of-use, which is already a big selling point of PaaS. Services like Matcher are the reason to use the cloud.
Sign up for the Matcher API's trusted-tester waiting list.