I compared the last 50 transactions relayed by my Electrum server with the list displayed at
http://blockchain.info/ip-address/78.47.154.42Results:
- accuracy: 36%
- false positive rate: 64%
Technically, it's not too bad, because the number of possible IPs in the network is quite large.
However, I don't think that such a high error rate makes the data useful for any purpose.
Maybe they should publish a list of n IPs that could have relayed the transaction, with a percentage of confidence.