A few days ago in London (Kentucky, not England) a woman was apparently bitten by a snake during a church service. She was dead four hours later (the story can be read here, among other places).
Most of us are at least aware of the practice of snake-handling in churches. It's more common in small rural churches than in, say, megachurches, but most of us are familiar with it, even so. There are a whole host of problems with it, theologically. For one thing, it seems to me that playing with venomous snakes in the expectation that God will keep you from being bitten stands in plain violation of the commandment to not test God (Deuteronomy 6:16, quoted by Jesus in Matthew 4:7 and Luke 4:12). From a theological standpoint, it seems that this ought to be sufficient to discourage the practice.
Also worth noting are the concepts of hyperbole and metaphor. It seems entirely probable to me that when when Jesus tells his followers they'll be able to handle serpents without being harmed, he may not have meant that they would perpetually impervious to snake venom (side note: the same passage mentions drinking poison without harm - do these snake-handling churches, say, put out hemlock punch during services?). There are lots of ways to interpret that statement that avoid the obviously problematic conclusion that Christians are immune to snakebite (though that would have made Snakes on a Plane a more interesting movie: "It's okay. I'm a Christian!").
Finally - and of no small significance - is something that is generally well-known among Biblical scholars, but somewhat less known among laypersons, namely, the questionable provenance of the concluding paragraphs of Mark's gospel. In order to avoid an overly detailed and technical explanation of the discipline of textual criticism, and of the specific details of this situation, I will simply say that the earliest and best manuscripts of the Gospel of Mark end at 16:8. The remainder of the chapter - vv.9-20, which include the bit about handling snakes and drinking poison - were added later, possibly being excerpted from a late 2nd century document which is now lost to us. Moreover, the oldest of those manuscripts which do contain the text mark it in some way as being an addition, whether with asterisks or scribal notes to the effect that the passage is spurious. What is likely the case is that at some point someone decided that the ending of Mark was too abrupt and needed some reference to Jesus' post-resurrection appearances, and so felt it necessary to tack on a fuller ending (such practices at that time were not entirely out of the ordinary, nor thought to be terribly dishonest - the concept of intellectual property did not exist then). This ending, whatever it's origin, managed to get adopted into the majority of manuscripts, and all indications that it was a spurious addition were eventually lost, with the end result that the Textus Receptus - the manuscript that underlies the King James Bible - runs seamlessly from 16:8 into 16:9. Also, in addition to the manuscript evidence, the internal evidence of vv.9-20 argues against the passage's inclusion with the text of Mark. There are significant differences of style and vocabulary in these 12 verses, including a number of words not found anywhere else in the document.
Now, this discussion does raise some interesting theological questions. How do we deal with the fact that the church for several centuries has regarded these verses as inspired Scripture? I'm not so much a Protestant that I'm willing to simply ignore tradition entirely. But if Mark, the inspired author, did not actually pen these words, can we be certain that they are, in fact, inspired Scripture? It's a sticky question with no clear or easy answer (the study of the Bible is full of such questions). Either way it has serious implications for the concepts of Biblical inspiration and inerrancy - both of which I affirm, for the record (as a professor once put it, "I just don't see a future in Biblical errancy").
Sticky theological and textual questions aside, though, I think we can safely conclude that anyone - however strong his or her faith - who picks up a venomous snake should not be surprised when they get bitten and die.
Also, as Jenny pointed out, this must be a difficult time for the poor woman's family. If they honestly believe that a true Christian ought to be able to handle snakes without harm, what must they think about this woman's eternal fate? Unfortunately, the people who practice such things in church are often also the type of people to reject the kinds of arguments I've made above in favor of a "Well, the Bible says..." Unfortunately, this traps them in something of a lose-lose scenario. If their theology is accurate, then this woman may well not have been saved. If it is, as I have argued, improper, then her life was thrown away doing something that Christians really ought not be doing. In either case, a senseless tragedy.