That could mean so many things! Wow, um...if it is true that he is alive, then my one theory was proven true But assuming it's not here's my opinions:

Maybe DD came back as a ghost, even though he may not have done so right after he died. Maybe he wanted to wait awhile, see how the school and Wizarding Community fared without him. If he does come back as a ghost, then he'll probably act as a ghostly Headmaster, with McGonagall acting as Headmistress, where he'll help Harry in his plight to survive and defeat Voldemort. Maybe he'll only come back until then and than leave for ever, only remaining in the portarit in he Headmasters Office.

If that doesn't happen (And I think it won't) then his portriat might be placed in the Great Hall, where everyone can see him and he'll be able to be with the students and help them in his own unique way. If that's the case then Dumbldore will no doubt want to sing the school song to show everyone not to give up hope, to keep dreaming of a world without violence and trife. By having his portrait in the Great Hall, it would show everyone he's not gone, he's still there.

Ok, thhat's about it for now. I'll keep hope until the book comes out. I still like my chocolate frog theory though!