I think it is most likely someone on one of the two fringe groups (left or right) is sending these.
It is possible that a leftist sympathizer would do this, but in the absence of information supporting this I generally take the direct approach. There are people who kill abortion providers. Therefore it is even more likely that there are lone individuals who are willing to scare them without doing any actual killing. Some guy living in his mother's basement probably got the idea from watching television.
We can also probably rule out PP itself, since a number of their places have been hit, not just a single target. I fear that a lot of people on both sides are going to try to take advantage of the current situation to terrorize others and to get their voices heard.
In the absence of information, the direct approach is the same as jumping to conclusions. Since the extreme left does have a history of commitng hoaxes to garner sympathy and support, you have to figure that into your thinking. If your going by what you think is most likely, then you should count up the number of cases of abortion violence over a period, and compare them to the number of hoax cases, you'll probably find there were more hoaxes than violence. So the direct approach would would be to label this a hoax instead of a case of abortion violence. I think either would be jumping to a conclusion.
We can also probably rule out PP itself,
You can probably count out the senior officers of the organization, but not the staff. Advocacy groups (of any any type) tend to attract people who are zelots for that cause. Who would have easier access to a list of Planned Parenthood clinics than someone associatied with the organization? As far as I know, the powder hasn't been confirmed as anthrax. If it turns out to be just a harmless powder, that would seem to strenghten the possibility it was someone trying to garner attention and sympathy for Planned Parenthood.