This gives her a bit of dignity, and will frustrate Gordon Brown who would have liked to announce this himself. The reshuffle is one of the few weapons left in his arsenal - with Labour's poll rating now below room temperature, he is about to lead the party into what will doubtless be its worst-ever defeat. And he'll look more helpless now; given he can't control his own reshuffles in a timely way.
Home Secretary is perhaps the worst Office of State, a place to dump your worst enemy. So I suspect Brown will put there one of his potential rivals for the leadership: Alan Johnson, David Miliband or James Purnell. Johnson would refuse, whereas Miliband and Purnell would accept - I reckon - judging the experience would be good should they ever need to make a leadership bid (now, or post-election).