Satoshi abandoned bitcoin the very same day Gavin Andresen informed him of his plans to visit the feds in Washington to discuss the project.

By examining the last posts of Nakamoto, you can clearly see he had no plans to give up on the project until then. I don't think had he planned on leaving, that he would have left everyone holding the "there are still more ways to attack than I can count" bag.

Fortunately a young Canadian programmer became interested in the unfinished work of Satoshi and began piecing together the parts that Nakamoto had yet to finish. He completed the 51% attack defense and integrated it into his own coin.

He continues to work until this day - in secret - continually improving his predecessor's code. The last public statement from this mysterious developer was that he planned to release the world's first multi-threaded modular java client in spring.