Taken at face value, the idea of sola scriptura is self-refuting and even unbiblical. I realize that I am not offering anything original by making such a statement, but this idea has kind of been developing in my mind as of late (influenced, in part, by an article I happened to stumble across by Tim Crosby in Ministry magazine from October, 1987, available here).
Taken at face value, sola scriptura implies that the scriptures, and the scriptures alone, serve as our rule of faith. We live solely and only by the scriptures. We do not allow for anything outside the Bible to influence how we live or what we believe. But, as I said, the phrase sola scriptura is itself self-refuting because no such statement is ever made in the Bible, nor even hinted at. In fact, places like Psalm 19:1, which says, "The heavens declare the glory of God; and the firmament sheweth his handywork," seem to imply something altogether different.
It was the Protestant reformers who made sola scriptura their foundational creed. Attempting to neutralize the Catholic practice of Apostolic tradition and a sort of sensus fidei (sense of faith), the reformers sought to return to the Bible alone to determine how they were to live. What they did not wish to imply is that science is therefore irrelevant, for example, or that history cannot teach us lessons.
The truth is, practically speaking, nobody that I know of practices sola scriptura, literally speaking. Just this morning, when I was interacting with a friend of mine who belongs to another community of faith, and I teased him about eating cheese, he said, "Which text tells us that we shouldn't eat cheese?" I responded, "The same text that tells us not to smoke." He conceded the point (though I doubt he feels convicted to give up cheese!) It is absurd to suppose that God does not continue to reveal truth to us that moves beyond what the Bible has already revealed. Very few of us would brush our teeth in the morning (or night, or whenever we do it) if we strictly adhered to sola scriptura.
Some may think it is kind of silly to even have this type of discussion. Of course we allow for outside witnesses beyond the Bible to determine how we live. But the discussion may not be all that silly. After all, such a "fundamentalist" attitude toward scripture and revelation can cause a somewhat condescending attitude towards nature and science, for example. Or it may close our minds towards the idea that, perhaps, God has revealed Himself explicitly to individuals after the Canon of scripture was closed (Ellen White, anyone?).
But there is caution, of course, and that is why we might be better off taking a prima scriptura approach. That is, we judge everything against the Bible, but allow for other avenues to determine how we should live. Yes, we take into account science, tradition, further revelations, but we do not accept anything that directly contradicts the Bible - and I emphasize "directly" because we may discover things in science, for example, that contradict behaviors that even Jesus engaged in, but that are not necessarily mandated in scripture. So, if we discover that eating lamb isn't the healthiest thing in the world to do, we can't necessarily go to Jesus' example at eating the passover lamb and say, "See, Jesus ate it, so we should too." Nowhere does Jesus give a command to do anything of the sort and so we are not to make it a rule to live by in our own experience. The Bible doesn't say that Jesus brushed His teeth, either, but this shouldn't lead us to believe that brushing our teeth is not important.
On the other hand, when it comes to something like practicing a homosexual lifestyle, for example, and modern psychology proposes that it is perfectly normal to practice such a lifestyle because a person's sexual orientation is "neutral," and therefore perfectly acceptable and even good to indulge, we have to reject such a notion because it directly contradicts normative and prescriptive statements in scripture (see 1 Cor 6:9, Lev 20:13, etc.).
Anyway, you get the idea, I hope.