Vaeflare over at the official forums posted a clarification about magic find on followers. Currently, MF caps at 300% with your gear, 375% with the Neph stacks and with any extra from your follower going over the cap. Blizzard says that the total cap is supposed to be 375% (with Neph) and that any MF over that will not count. Even if it's on your followers. So don't spend those crazy prices on the AH to gear them, because it would be a waste.
Here is the quote:
Blizzard tracker: August 24, 2012
The Magic Find and Gold Find percentages for both follower contributions and Fortune Shrine bonuses are intended to respect the 300% MF/GF cap. Nephalem Valor bonuses will continue to stack beyond this cap, however.
That said, we’ve seen some posts that MF/GF contributions from followers may currently be stacking above the cap, and we’re looking into those reports.