Just in case, if you have a Canon 40D/5D/1D/1Ds, it's a bit simpler than all that, since the Canon higher-end bodies are made to have replaceable focus screens (I'd assume the higher-end Nikons do, too).
Replacing the focus screen on a 40D takes about 30 seconds. If only the entry-level bodies made it so easy....
The three major caveats I've heard about the split-focus screens is that 1) AF confirm signals may no longer be visible, 2) for lenses with max. apertures smaller than f/2.8 you can get viewfinder blackout (particularly dark with f/5.6 superteles), and 3) spot metering will not be accurate if you use the center AF spot.