link building

How does Google weigh forum posting in considering backlinks? Can anyone tell me?

The same as any other backlink that is from an irrelevant source.

Similar to blogs. Say you put your sig link on 100 indexed posts, it would be silmilar to having your link on 100 indexed comments of a blog. Same ip for all the links, but if you are lucky some of the posts will be popular and get linked to a lot and you will get some of that link juice flow and maybe a little PR.

If forum postings have done with relevant contents and textual link then it has great value.
To get better results you can ping them.
If forum posting has irrelevant content then count as simple backlinks have lower value.