STL container assignment and const pointers

This compiles:

int* p1;
const int* p2;
p2 = p1;

This does not:

vector<int*> v1;
vector<const int*> v2;
v2 = v1;  // Error!
v2 = static_cast<vector<const int*> >(v1);  // Error!

What are the type equivalence rules for nested const pointers? I thought the conversion would be implicit. Besides, I'd rather not implement point-wise assignment of STL containers, unless I really have to.