Boston is in. They could still be WC2 or WC3. If they win one of two against Detroit, they're WC2 and will play either NY or Tor. If they lose both, then it depends on Cleveland. Cleveland owns the tiebreaker over Detroit and Detroit owns the tiebreaker over Boston, so a three way tie (Det wins 2, Cle wins 2) would give Cleveland the division, Detroit WC2 and Bos WC3. If Detroit wins 2 and Cleveland wins 0 or 1, Detroit wins the Central and Boston is WC2. WC3 would come down to what Houston does (1 game back of Det/Cle right now).
I can't imagine Boston trying very hard knowing they're going on the road regardless and there doesn't appear to be much difference between NYY/Tor/Cle. So Boston clinching is good for Detroit and bad for Houston.