With the nature of social media nowadays, some sites get thousands of incoming links in a day. Get a huge news article on the front page of digg and the bloggers go nuts and the links explode. So in that respect, Google wouldn't automatically penalise a sudden burst of links imo. On the other hand, if your link suddenly turns up on 2000 pligg sites through some spam tool, Google would be more likely to boot you.
Do not build as many as you want. You may want 1000 links for a domain started yesterday, but it will promptly blacklist you. the link building should be a natural process done over months. You should have 1000 quality links one month, 2000 the next, and so on. If you start a domain today, add 2 links today, and 1000 links tomorrow, consider yourself banned.
I think that as long as you are building the links yourself then you would not have to worry about a penalty. If you buy 1,000.00 back links for 5 bucks then I guess you get what you pay for. Since the exact information is not released on how the search engines take all of this into account, I guess a little common sense will go a long way.