Honestly, I find this entire topic confounding from a Theravadin standpoint. To begin with, you have people who declare Theravada Buddhism to be a philosophy, religion, or ethical way of life free from any form of discrimination. These people claim that everything the Buddha taught was true, and that everything he said is exactly the way it is. Moreover, these people often state that the commentaries faithfully represent the Buddha's message. Whatever he said, whether it appears to contradict this assumption or not, must be taken at face value. Therefore, teachings that state a woman is incapable of becoming a Fully Awakened Buddha are taken to mean that women are incapable of ever achieving this level of spiritual attainment, and at least one commentary states that, "The masculine sex is superior, the women inferior" (Pe Maung Tin, The Expositor, 420).
What I find so bizarre and contradictory is that the assertion is made that gender is nothing more than a conventional distinction, one that contains no inherent meaning or reality; however, you find teachings stating that "women" are unable to do something spiritually that "men" are capable of doing. If this is true, then for what reason is a woman, or a stream of consciousness in the form of a woman, unable to do become a samma-sambuddha? If it merely depends upon your level of accomplishment through aeons of effort, I do not see what would prevent a woman, or a stream of consciousness in the form of a woman, after developing the perfections and being reborn countless times while doing so in all the realms of existence, finally taking rebirth as a female and bringing back the Dhamma to the world after it has died out completely.
It could very well be, as Peter has suggested, that the reason the Buddha said this was due to the societal conditions at that time. Perhaps the Buddha himself never said that women are inferior to men, or in other words, that being born as a woman is inferior to being born as a man; nevertheless, you have commentarial literature stating that this is indeed the case. So even if the Buddha himself did not say this, that is the way it is interpreted in the "classical" Theravada Tradition in which the entire Tipitaka and its commentaries are considered authoritative. My opinion is that an element of sexism may have found its way into the interpretation of the Buddha's words recorded in the Pali Canon, and that this is not as unlikely as it sounds considering that these teachings have been transmitted through a predominately patriarchal society for almost 2,600 years.