According to Adam Boulton in the Sunday Times today, IDS (or GIDS, as Boulton calls him because his first name is George; so that's where Gideon comes from!) experienced what Boulton calls "a Damascene conversion" after a visit to the deprived Easterhouse estate in Glasgow. Apparently it was after that visit that GIDS "reinvented himself" and founded the Centre for Social Justice.
If this is true, then maybe the resignation isn't all that out of character after all.
Other comments in the same newspaper reckon this has given Boris a clear run for the Tory leadership, and subsequently prime ministership!
Time will tell.