Funny, it doesn't actually bother me to have a line or two (usually only that many) repeated at the top. In fact, it's especially useful for things like tables, so you can see the connection between what was above and what's below and not risk overlooking a line. If they didn't repeat the line, I would wonder if I had missed seeing something, if I had maybe clicked twice on the scrollbar instead of once.
For some reason I've never tried scrolling with the mouse wheel, and I don't drag the side scrollbar down, either. I think it's that I find discrete chunks of text easier to take in at a glance -- once it's all moving too fast, then you really have lost your place and you can't easily scan to find it.
Scanning behavior seems to operate sometimes on a less than conscious level, though. Have you ever laid down a book or magazine article, gone into another room for a while, then come back and found your eye going to exactly the next sentence, even though you didn't consciously remember where you left off? I often go back and reread the previous sentence just to be sure I didn't miss anything, but very often, it's exactly the one I had just finished. Amazing, but if you had asked me to tell you what it was, I couldn't have said.
eric, have you never penciled in a note or two of music to cover an awkward page turn, just because it helps to see the continuity, the relation of one measure to the next? Like when it's a less than predictable interval, and it helps to see the note at the bottom of one page and at the top of the next in relation to each other, so your mind can interpret it as an interval? (Unless you have perfect pitch ... (-; ) To me it's similar to that -- seeing more of the context actually makes it easier to read faster.