Amazon announced a bunch of new instance types today, and some of them look really awesome for builds.

The new C3 instance types are available now, and they have "per-core performance that bests that provided by any of the other EC2 instance types, at a price-performance ratio that will make them a great fit for many compute-intensive workloads". They also have SSDs, which I think will make a lot of builds much, much faster than with traditional EC2.

http://aws.typepad.com/aws/2013/11/a-generation-of-ec2-instances-for-compute-intensive-workloads.html

Also, they have an i2 instance type coming out soon which is an absolute BEAST: http://aws.typepad.com/aws/2013/11/coming-soon-the-i2-instance-type-high-io-performance-via-ssd.html . Would be amazing to be able to run these machines as well, although I think c3 is more important.