EPT1) Your league after placement matches depends not only on your record, but on the strength of the opponents you face. 3-2 can lead to both Gold and Silver, for example.
2) After every 30 games, your account is examined for promotion/demotion. I'm absolutely certain it worked this way at least two times in my case.
I'm sure this covers all "promoted after loss/demoted after win" scenarios. Please take into account that there are some other milestones before first 30 games.
3) The system seems to use some sort of "hidden rating", which affects the level of players you face. This allows relatively fast promotion if you get placed in wrong league.
Also there was some bug with favored points after last patch. Even though I had only 4, I was receiving 17+ points for a win and only 4-10 for a loss, so I had just to play many games to reach Platinum that time. I think Blizz fixed it server-side, saw it mentioned somewhere.
