Looks like it will be coming down Wednesday or Thursday.
For whatever it's worth, SCOTUSblog's Tom Goldstein reports that "The only opinion remaining from the March sitting is Heller. The only Justice without a majority opinion from that sitting is Justice Scalia." Since usually each Justice is assigned at least one majority opinion per sitting, that's something of a sign that Justice Scalia is likely to write the majority opinion in Heller. But it's of course far from certain, for a variety of reasons.
For more on how this guessing game is played, see this SCOTUSblog post from last week. Note that it says neither Justice Souter nor Justice Scalia had majority opinions from the March sitting, but Justice Souter wrote one that came down today, so that's why Tom Goldstein says only Justice Scalia is left.