Old question already answered, but since the game has seen some changes since, an update seems necessary for the newcomers:
Currently in patch 1.0.7, the hard cap on Magic Find is 625% (300% soft
cap + 75% NV stacks + 250% MP 10 on Inferno).
Starting in patch 1.0.8 however, the Magic Find bonus from shrines
will be able to exceed the “soft cap” of 300% that players can achieve
from a combination of their own gear and their follower’s gear. In
patch 1.0.8 we’re also adding a buff to multiplayer games called
“Strength in Numbers” which allows for a 10% bonus to Magic Find, Gold
Find, and experience per additional player in a multiplayer game, up
to a maximum of 30% in a four-player game. The Magic Find and Gold
find bonuses of this multiplayer feature can exceed their respective
300% cap as well.
So after patch 1.0.8, if you have 300% Magic Find from your gear and
your followers, have 5 stacks of NV (+75% MF), are playing on Inferno
at MP10 (+250% MF), have a Fortune Shrine buff active (+25% MF), AND
are playing a four player multiplayer game (+30% MF), you can hit the
maximum of a 680% bonus to Magic Find. Shiny!
Keep in mind that this cap will be lower for if you aren’t playing on
Inferno and/or aren’t using Monster Power level 10, etc.