Not really. There are always lots of 'versions' of the blockchain floating around. The network only keeps track of the longest chain (broadly speaking). This means the network with 51% can determine which transactions get into the blockchain. For example, if the pool owner doesn't like you, he can essentially 'blacklist' your account, which means your Bitcoins can become unspendable, basically. The longer they have the 51% power, the more damage they can do. GHash already performed what you describe as your '49% attack' against a gambling site that accepted 0 confirmation deposits. There has been no known instance of a 51% attack yet (e.g. double spend after 6 confirmations).
