Currently, Cython does runtime checking for certain types (e.g. lists and tuples) for things like fast indexing. This speeds things up immensely. However, sometimes it would be nice to declare a type so the fast methods would always be used. With type inference, this could be especially useful (e.g.  would know it's a list which would be propigated when one calls append).
I (robertwb) propose that
cdef list L = x
be the syntax, and it would raise a type error if x were not exactly a list (or None?). Subclasses would be disallowed (one could not safely access their members through non-generic methods) and conversion would not happen (it could be prohibitively expensive and is easy to do explicitly).
How to name Python ints/longs is an interesting question.
A proposal for extending this to all Python types can be found here. It didn't get support due to the fact that it requires syntax that completely contradicts all common OO principles which will be very confusing. It can be altered to support OO principles, however it then doesn't bring any performance benefits and seems to be too much trouble only for the sake of some consistency in the language that nobody will use.