CPU time does not scale as well with bitcoin mining as GPU time, because it deals with hashing algorithms which work better in parallel. This indicates that the best existing device for this purpose is the Radeon ATI 5970 graphics card, which is so extremely parallel that it has 3200 stream processors. This indicates that four of these can be placed in a single computer at a low cost if you know what you are doing, and are dramatically cheaper than Amazon's EC2 cloud computing resources. (I've noted that the author of whitepixel also hangs out on the various bitcoin channels on freenode.)