Hey can someone answer this stupid question for me please! How can I get particles to emit from where I weight paint in the new particles system?! I tried everything nothing works. 
THanks!!
Have you created a vertex group and assigned the vertices?
That should give all the vertices an equal weight.
Test with something that has lots of vertices - say a 32 * 32 Mesh grid.
In the Object Buttons Window > Extras panel > near the bottom to the right of the ‘Neg’ label, there is a little arrow to select the name of the Vertex group. Make sure you set it to your vertex group.
Set the particle type to ‘Hair’, bump up ‘Nor’ a bit.
Then go to weight paint mode, adjust the weight in the Edit Buttons window > Paint pane; and you should see the density distribution of the hair vary as you weight paint.
thanks! I was looking for it but I it is really difficult to find it took me about 7 minutes to find is after I read your comment.