That that amount of ricin could kill 3 or 4 people. I'd think that if mixed into pounds and pounds of meth, it wouldn't do much, if anything. I initially thought that too...but I just don't see that happening. I see the following as potentials:

1. Ricin in the cigarette - for Jack, likely.
2. Ricin in the tea/Stivia - for Lydia - though Walt really has no issue with her (he doesn't know she threatened Skylar, and probably hasn't had contact with Skylar before he gets the ricin)
3. Ricin for himself - to guarantee he won't live long once put in jail.
4. Ricin for Jesse, somehow.

I really don't think that his old partner's even a possibility, really.