Documentation.

docs/contracts.rst

@@ -447,6 +447,11 @@ In particular, the following operations will consume more gas than the stipend p
 
 Please ensure you test your fallback function thoroughly to ensure the execution cost is less than 2300 gas before deploying a contract.
 
+.. warning::
+    The default fallback function will throw an exception (this was different
+    before Solidity v0.4.0), so if you want your contract to receive Ether,
+    you have to implement a fallback function.
+
 ::
 
     contract Test {