With books titled "Hagakure: The Book of the Samurai" and "Budo Shoshinshu: The Code of the Samurai" flooding the market, one generally comes to think that the samurai ALL followed the Bushido, or else they weren't really samurai. This just isn't the case.
Bushi-do is correctly translated as "The Way of the Warrior". However, the Bushido was nothing more of an invention of the Edo Jidai (Edo era) meant to keep samurai subservient to their employing daimyos. The Edo Jidai was the 250 year long peace ruled over by the last shogunate (Tokugawa Shogunate), which directly proceeded the Warring States era (better known as the Sengoku Jidai). The shogunate was very paranoid during this period; it was, after all, the third shogunate, the first two having collapsed into warfare. Several new practices came into play during this time period; for example, daimyo were expected to spend part of their time in Edo (now known as Tokyo), the seat of the shogunate's power. This was to keep unruly daimyo (like Shimazu or Mori daimyo, who would later tear down the shogunal government) in check.
Another practice was a serious enforcement of a samurai's loyalty to his daimyo. During the Sengoku Jidai, there were several instances of samurai turning on their daimyo, most often to disastrous affects. The shogunate made turning on one's daimyo the most serious offense for a samurai.
Also, during this time of peace, the samurai no longer had as much purpose in Japanese society. They became administrators and small time government officials. With this pretty much 'excuse' for existence, a ronin named Yamaga Soko took the Confucian principals that had been governing Japanese life for centuries and gave the samurai a new reason for existence. Soko's codices later became the foundation for the bushido, which received great support from the shogunate. The shogunate was looking for ways to insure that the samurai did not rise up against their daimyo. As one of Soko's rules was a complete, sincere devotion to one's feudal lord, this fit perfectly into the shogunate's paranoid attempts to suppress all possible revolts... and it worked.
Samurai before the Edo Jidai did NOT follow any "Bushi-do", then. In fact, there are so many examples of betrayal, uncouthness, and other acts completely contrary to the bushido in the Sengoku Jidai that it becomes glaringly obvious. This isn't to say however that there weren't noble samurai. In fact, Uesugi Kenshin is renowned for his honor in his battles with Takeda Shingen in the late 16th century.
This information comes directly from the Samurai Archives. A great source for information about the samurai and other aspects of Japanese history prior to the Meiji Restoration.