Objective-C++ 11 - Why can't we assign a block to a lambda?

So, I just upgraded to Xcode 4.4, and I noticed in the changelog:

Apple LLVM compiler supports additional C++11 features, including lambdas

Which is awesome! So I got around to coding, and I found a few things out:

1. Lambdas are assignable to Objective-C blocks:

   void (^block)() = []() -> void { 
       NSLog(@"Inside Lambda called as block!");
   };
   
   block();
   

2. std::function can hold an Objective-C block:

   std::function<void(void)> func = ^{
       NSLog(@"Block inside std::function");
   };
   
   func();
   

3. We cant assign an Objective-C block to a lambda:

   auto lambda = []() -> {
       NSLog(@"Lambda!");
   };
   
   lambda = ^{ // error!
       NSLog(@"Block!");
   };
   
   lambda();
   

Why is this? Shouldn't the two be semantically equivalent, given what we've seen above?