That depends I suppose on the situation, and its highly subjective.

For example, if you continue to push a product in several threads where the topic(s) are for different products, you're probably crossing the line.

If the thread is about a particular engine and its merits or downfalls, bringing up another engine with features the people in the thread might like to have would be acceptable. "Engine XYZ has what you're looking for..." would be acceptable.

I guess I would say it's the intentions of the post(er) that dictate whether it crosses the line or not.