From the results of the contest, I would have concluded that the platform of choice was CUDA. Do you think otherwise? Is there a reason you'd choose to use a flotilla of Rackspace servers for a problem like this rather than a graphics card? If you are going to be programming in C anyway, it seems like a much simpler approach.
I think using cloud resources was the way to go though, because it's easy to search for keys concurrently, and you can summon an effectively unlimited amount of computing power pretty cheaply. You can add as many cloud servers as you want to your cluster without worrying about locking or divvying up work, and the number of keys your cluster is able to check per second increases linearly with the number of machines.
The winning team, awesome guys with a cluster of 8 very powerful computers and a CUDA program, were able to check just over 400 million keys per second. If I would have used 400 Rackspace cloud servers, it would have cost me about $100 to run them for the duration of the contest and I would have been achieving similar speeds.
"With all the machines running we got up to a total of 3079431974 hashes/second" (via http://www.win.tue.nl/cccc/sha-1-challenge.html)
Presuming your costs are accurate, this would imply that the card would be a cheaper solution if run for more than a week. Considering the ~100x speed advantage of the card, it's rather surprising that their cloud solution is as affordable as it is.