I'm torn two ways on this - logically Rumsfeld should resign, and if he doesn't, it dramatically lowers the standard for what can be expected of public servants. (In the same way that Clinton did by hanging on.)

On the other hand, if he stays, that makes it worse for Bush, because it ties the responsibility more clearly to him.

So I think he should resign, but I'll be rather glad if he won't, and if Bush refuses to sack him. But I think that, sooner or later, one of those things will in fact happen. And the more it is delayed, the more harm it does Bush. So that's OK.