I can fit 2 Mac Minis in a 1RU. And that is a Core i7, 16GB, 2xSSD which is about 20-30x faster than what Amazon, Rackspace offers for the same price. Now throw in the fact that you can (a) send it back to Apple if it breaks or (b) sell it on eBay for nearly the same price it all becomes very compelling.
Given that I use a Java/Hazelcast/Cassandra stack I can simply add more Mac Minis to linearly scale with no single point of failure.