I read about SeaMicro a year or so ago, they are a hardware company in Silicon Valley. I've been through the pain of trying to do hardware development in a previous job, and trust me its hard. Its not like software development where you can just recompile, with hardware it costs real money when you make mistakes.
These guys have managed to get 512 cores into a 40U rack. They are saying four times the compute density at at quarter of the power. They have a 1.28 Tbps interconnect between them all as well. I would love to play with one of these. I can see web companies with there own data centers getting very excited about these.