How does the Monster Power bonus experience work with the normal experience bonus?

An updated to this post, since it not accurate anymore:

As of 1.0.7, the experience bonus gained from Monter Power level as been change to the following and everything is additive:

• MP 1: 25% (up from 10%)
• MP 2: 50% (up from 20%)
• MP 3: 80% (up from 30%)
• MP 4: 120%(up from 45%)
• MP 5: 165% (up from 60%)
• MP 6: 215% (up from 75%)
• MP 7: 275% (up from 95%)
• MP 8: 340% (up from 115%)
• MP 9: 420% (up from 135%
• MP 10: 510% (up from 160%)

Source

For the next patch 1.0.8, Blizzard said Valor Stacks will be multiplicative, like the 10% bonus gained for every other players in a multiplayer game (hence limited to 30%):

Also, just reiterating what others have said, Nephalem Valor stacks are now multiplicative, not additive. The simple calculation is: [ 100%(base) + bonus from MP + helm ruby + ring] * [1 + Up to 0.75 from NV + Up to 0.30 for multiplayer]

Source

Experience bonuses are always additive, that is, if you are playing on MP5 （+60%) with a radiant star ruby in your helmet (+31%) and a hellfire ring of +35%, you gain 226% (=100%+60%+31%+35%) experience of the experience when playing on MP0 without ruby or ring.