Another false argument going on seems to be that Goku's source of invulnerability is his "Ki", or some sort of shield produced by that. Although this is the current incarnation of Superman's form of invulnerability, this is not the case with Saiyans.
During Dragonball Movie #14, Goku is shot by Bulma in the head. Now, while Goku is already a formidable martial artist, and fighter, he has not yet met Master Roshi. He has not learned to develop his "ki" in any significant way. He has not yet mastered the kame hame ha. However, the bullet ricochets off his head, and apparently the bullet only stings the child Saiyan. Goku certainly doesn't have the ability to produce a field yet, as he has no experience with ki-type attacks. Moreover, if it were a force field, why would the bullet sting? It would only bounce off the field, and have imparted none of its energy to the Goku's head.
Also, we can tell that Goku's invulnerability is not solely due to a force field by comparing it to Superman's invulnerability. In the comics, Superman's current incarnation has his clothes made out of normal materials, not some Kryptonian fabric as was the case PreC. His threads stay together because of a force field that extends a very short distance from his body. So, when hit by a particularly brutal attack, Superman's cape often disintegrates, but the suit, only a fraction of an inch from his skin, remains intact. Conversely, how often do we see Goku's shirt getting blown off of him? Every third episode? And yet, Goku seems to survive these attacks that penetrate this force field. Clearly, Goku himself is fairly invulnerable, and this is not solely due to a "force field".
However, it is true that characters in Dragonball do produce some kind of field. This is seen in countless episodes, as debris, smoke, and energy attacks are seen to come in contact with these fields. However, this is merely a supplimental form of protection, and the Saiyans (and most of the characters in the DBZ universe) are also physically tough...very tough.