The IDS to Defence rumour has been doing the rounds for a few days - the problem being that IDS seems to be reluctant to leave DWP, and it seemed odd that Hammond would be on his way out. Now Hague has gone, giving a promotion opportunity, that gives a degree of sense to the suggestion that IDS would move.
I imagine that a reasonably big-hitter will move into the post, since the risk of the Torygraph and the Mail banging on about not taking defence seriously if a relatively junior minister got the job (think Ainsworth's appointment).
If rumours of his return to government are correct, the 'obvious' answer would be the rehabilitated Liam Fox, who might manage to master his brief second time around. God help us...