The Particle Interaction Panel, in the Object context buttons, now has a couple of new options available. Fields and Deflection work on Particles, as well as the new Soft Body feature.
Typically, an Empty Object is used to add fields. Only mesh objects can be used for deflection (collision).
Wind Field
A new field type is "Wind", which acts as a constant force in the Z direction of the Object. You can control the force with "Strength" and "Fall-off", which are availble as animation channels in the Ipo Curve Editor as well.
Note that "Wind", unlike the other Field types, requires a Soft Body to have global Friction set. With zero friction, a Soft Body won't be affected by wind.
MaxDist
For a more precise control of Fields, you can denote a maximum distance for the field to affect. This isn't available as animated channel yet.
Soft Body Deflection
If Deflection is set, Soft Bodies can collide with any Mesh Object. Since the current collision detection code is relatively simple, collisions are sometimes slow and unprecise. Soft Body collision is currently in a beta stage.
The "Inner" and "Outer" values define a force within Mesh faces that push away Soft Body vertices. The "Outer" value is in the direction of a face normal (front side of face), the "Inner" value for the back side of a face.
The values themselves are in Blender's 3D units; the effect works only within this distance.
Note: only non-soft Objects can become a Deflector. You cannot have two soft Objects colliding with each other.
You can optimize speed of Soft Body deflection by creating simple helper Mesh Objects in an invisible layer. Make sure the Soft Object then also has that layer set.
Wind and deflection code: Jens Ole Wund