particle play ground will let you use a layer map to effect particles... and one way would be to use the layer map to effect particle speed and have them 'stick' to the layer map. you would need to create a layer map that looks like the parts of the trees that snow would stick on (branches and such).
here is a tutorial that shows how to set that up:
http://library.creativecow.net/articles/drozda_jerzy/ParticlePlaygroundOnIc...
particular (from trapcode) is a particle system that will allow you to define a layer as a wall or floor and use the layer's alpha to effect particles. you'd set the physics model to 'bounce' then set the plane (either floor or wall) to the layer that you want to define the areas where snow would stick, and set the collision event to 'stick'. then particles that hit those areas will 'stick'.
Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW
