There has been no official confirmation on who it is yet, but if you head over to BTCGuild and check on their Hall Of Fame page you will see a user, called 67117 that is mining at a ground breaking 800 to 900 GH/s (or 800,000,000,000 to 900,000,000,000 Hash's per second, or .8 to .9 TH/s if you prefer).
This represents nearly one fifth of BTCGuild current hashing capacity and nearly one twentieth of the total capacity of the bitcoin network
There is a lot of speculation by the community on BitcoinTalk in the ASICMiner thread that the rack ASIC bitcoin mining devices that we saw pictures of earlier this week have finally found their way onto the network, but as I say, no confirmation yet.
It does seem though, the time of bitcoin ASIC has finally arrived, even though it is only for a few Avalon ASIC owners and whoever this user 67117 is.
It is probably summed up by BTCGuild operator Eleuthria who earlier said in a discussion on #btcguild;-
. now we have a single user with 800gh? 0_0
Welcome to the future.Eleuthria
UPDATE: By the way, whoever user 67117 is they will be generating about 122.83 bitcoins a day at the rate of 800 GH/s, or \$3,000 US Dollars at today's rates.
UPDATE2: It is indeed ASICMiner.