When a Justice recuses from a case, that is announced when the Court announces that they voted to hear the case, because the recused Justice doesn't vote on the decision to hear the case. (That is why Kagan's recusal was announced today, together with the Court's decision to hear the Arizona case.) Kagan did not recuse from the vote to hear the Obamacare cases.
Can a Justice later recuse from a case, after the decision has been made to hear the case? I would expect that to be true, hence the growing pressure on Thomas and Kagan regarding ObamaKare