I got this email too:

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Authorize log-in attempt

An attempt to login to your blockchain.info wallet was made from an unknown browser. Please confirm the following details are correct:

Time: 2013-04-26 22:03:19
IP Address: 46.167.245.50 (Czech Republic)
User Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8) Gecko/20051111 Firefox/1.5

If the above details are correct please use the following login link:

https://blockchain.info/wallet/

If this login attempt was made by you this email can be safely ignored however you may wish to change your wallet alias.
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I don't have any coins there, so good luck with that. However, I used a common word as the wallet identifier, as some other people here apparently did. I am guessing someone is blindly trying weak identifier/password combinations.