For folks wondering -- why yes, you can scan the entire blockchain for repeated k values. It's about O(n^2) for an N so small as to be effectively constant (n = outgoing transactions per address), and will be dominated by the time it takes you to actually download the blockchain.
...which appears to be how the problem was brought to public attention. Someone was draining balances held by keys so compromised, within a few hours after the repeated 'random' value.
So anyone new jumping on this would have to race the one or more existing exploiters.
