I don't think Hassert should resign now, but he should be replaced after the election.
Speaker of the House: Dennis Hastert (IL)
Majority Leader: John Boehner (OH)
Majority Whip: Roy Blunt (MO)
Chief Deputy Whip: Eric Cantor (VA)
If Hastert must resign they all must, the leadership should fall together. No chance of coming back to a leadership position, but if their constituents reelect them they can serve as a member.
Now the question remains if the FBI didn't have the IM's and they have the power of subpoena to get all records (paper, computer, wireless, etc). How did they miss them?
Then, who had the IM's? Any Republican? Any Democrat? This is the next head to roll. Should it ever be found that the Dems knew of it and "saved" it for the right moment, just prior to an election ... well I guess that the same as the dead voting for their candidate.
Hope something will happen to these politicians to truly wake them up ... but I'm not holding my breath.