Yes. Most recently in July 2014, GHash.IO exceeded 51% of the hashrate. It didn't perform a 51% attack, however, and quickly voluntarily reduced its mining power, vowing to stay below 40% in the future.

It's worth noting that in July 2014, the hashrate of the entire network was around 100,000 TH/s, meaning GHash.IO controlled around 50,000 TH/s. Current hashrates are around 1000x higher at 100,000,000 TH/s, so the amount of power GHash.IO controlled back in 2014 is currently equivalent to 0.05% of the hashrate. Performing a 51% attack now would require enormous amounts of hash power at a huge cost.

Prior to this, when the overall hashrate was even smaller (and so obtaining 51% of it easier), mining pools Deepbit in July 2011 (thread here: https://bitcointalk.org/index.php?topic=26656) and BTCGuild in March 2013 (thread here: https://bitcointalk.org/index.php?topic=152296.0), also crossed the threshold.