What is clear from the text of Genesis is that the snake didn't actually say anything that was untrue. I have just read an example of a scholar--not a biblical scholar, but an evangelical patristics scholar--who seems not to recognize this, so I thought I'd point it out, though I know it's been done many times before.
As I read Gen. 3:4-5, the snake says to the woman that four things will result from her eating the forbidden fruit: she will not die, her eyes will be opened, she will be like God, and she will know good and evil. (Probably those last three things are more-or-less identical to each other, but they come up at different points in the subsequent account, so I'll separate them.) This all happens just as the snake says.
- The narrator of the story tells us that, indeed, as soon as Eve ate the fruit and gave some to her man, "the eyes of both of them were opened" (3:8). Score one for the snake.
- At the end of the story, the narrator records God's words that the man [and woman, presumably] "has become like one of us" (3:22). The snake is right again.
- God also says in the same verse that the man [and woman] now know good and evil (3:22). That's three for the snake.
- Last, the snake promised that they would not die. Now, the text does not explicitly say that this did not happen, but all we have to do is read the whole story to realize that the punishment described in Gen. 2:17 (quoted by Eve in 3:3) was not carried out. The man and woman ate the fruit and did not die "in the day that [they ate] from it." The snake gets a perfect score.
I certainly have no interest in impinging on the truthfulness of God. Nor do I think the author of Genesis 3 would really want God to come out as the less truthful one in relation to the snake. But I think we must recognize that his words of 2:17 did not come to pass. I say this because I trust scripture (see previous post). My inclination is to hold together the truthfulness of God and the non-fulfillment of 2:17 by the assumption that God changed his mind--not an uncommon thing for God to do (see Jonah 3), though I do admit that there are some problems to this approach. But whatever we do with Genesis 3, let's at least affirm what is explicitly there in the text--the snake was right, nobody died, their eyes were opened, they became like God, and they knew good and evil.