Where to act on each new post?

[inxil]

I'm trying to write a little mod that will do something each time a new post is created. Originally I thought I could hook into postdata_postsave, but it seems like that's only getting called on replies—not on the first post of a new thread. In the end I need to add some data, along with a postid, for each post. Where would be the best place to do that?

Thanks!