Conventional wisdom states that you should start with the larger muscle groups first--in this case back, before working smaller muscle groups. When you work your back, you will work your shoulders and biceps as secondary muscles in some of the moves. If you work these smaller muscles first, they will be tired and you may compromise your back work. It also helps to preexhaust the smaller muscles. The back is a large muscle group and should get the focus of your energy.
That said, I have a vague recollection of reading an article that said something about how exhausting the smaller muscle groups will force the larger muscle to do all of the work. But I think this is really not for the average lifter, but may be a technique truly advanced body builders may try once in a while.
Still, isn't there a premix of back and biceps that alternates each, thus going from a bicep exercise to a back? This obviously must be ok if Cathe made the premix. I suppose it is appropriate if you are not focusing on gaining maximal back strength, say if you are looking for endurance or are focusing more on bicep strength.
If you were just doing shoulders and biceps, I think it would be ok to do bicep before shoulder.