That surprises me a bit william. I'm no maestro meself, but manual focusing is not that difficult. If you shoot in zoos at all, no matter how fancy the gear, you're going to need to manual focus past the wire fences. At short focal length, it's no sweat at all.
My apologies if I've misunderstood.
Good shooting with your D90, never had one but was always a bit envious when I had the D200.
It probably is easier than I make out. But on my D40, I found it really difficult. Perhaps because the viewfinder is smaller? I could get sharp shots, but not like I did with autofocus. Maybe I will investigate more when I get my D90. But again, im rarely unsatisfied with AF results, and if I am I simply retake.
Also, (dumb comment) with fences, why is this an issue? surely you just focus through the gaps, or lock it on an AF point beyond the foreground fence? Never found this an issue and I did this quite a bit when watching friends play tennis with my D40.