The question now is how Brown engineers the situation. Does he downplay the expenses aspect to Smith's downfall, and make it seem like this is all just part of a normal reshuffle? Or does he spin it as him "doing everything it takes" to clean up politics; portraying himself as a crusading figure who is even prepared to take out the miscreats on his own side? Much depends on the manner of Smith's departure. But if Brown takes the latter route, then you imagine Smith will - like the other receipt offenders - have to be denied the opportunity to defend her seat at the next election. And it will prompt questions about what Brown does with other ministers, such as Alistair Darling and Geoff Hoon.
The potential for a near unprecedented purge of the Cabinet is certainly lurking in the wings. But I think Brown will struggle to portray it as him taking affirmative action. He has already been compromised too much by this entire scandal, and if he places his acolytes in positions of greater power, then he risks inflaming tensions even more. In the end, I imagine all this will just further damage Brown's chances at the next election. If he gets that far.