Partially implemented.
Some things are checked and others aren't (mainly classes/objects).
Some builtin types can optionally take type parameters, which are erased at runtime. Other types are currently unable to take type parameters.
All Neko builtins ($<name>) are typed to some degree, with select functions like $apply, $closure, and $array having special/generic behavior.
Other functions and methods are currently unable to take type parameters.
Basic flow-typing is available is via switch $typeof(myVar) {...}.