The thing I really like about Amazon's cloud stuff is they're constantly undermining themselves with new innovations - spot instances are another great example. Taking the utility metaphor a step further you can now rent their services when nobody else is for cheaper, like buying electricity at night.
A lot of the tasks I'm envisioning for Sproozi aren't really time dependent. While it's important to show you a page in a timely manner, crawl and index a brand new website you add quickly and basically be interactive there is also a lot I have to do in the background. The huge and growing list of URLs people add all need to be re-crawled and re-indexed regularly is just one of many examples of processing vast amounts of data. These tasks are always running, always in the background.
Spot instances are a perfect fit - I can bid the price I want on extra capacity spin up some extra instances to join the cluster when they're cheap. Over the next few weeks I'll probably try to add some spot instances to my Hadoop dev cluster and see what happens.