Take slicehost or S3. To give you more RAM means that someone else has to need more CPU to make their virtualization strategy work. I think once Amazon got enough volume it could begin offering high CPU and high RAM instances without diminishing overall utilization efficiency.

But Amazon still charges quite a premium for high RAM.. probably b/c due to IO rate limits low RAM also results in less CPU burn for many workloads than would exist if there were plenty of memory available.