My vote would be to name it IsLast and change the functionality. If that isn't really an option, I'd leave the name as IsNotLast.
I agree with Code Complete (Use positive boolean variable names), I also believe that rules are made to be broken. The key is to break them only when you absoluately have to. In this case, none of the alternative names are as clear as the name that "breaks" the rule. So this is one of those times where breaking the rule can be okay.