Is there a setting in ReSharper 4 (or even Visual Studio itself...) that forces a warning if I forget to wrap code in a using block, or omit the proper Dispose call in a finally block?
Correct automatic Dispose analysis requires DFA (Data Flow Analysis) in a global way. It is unlikely that you create an IDisposable object and doesn't call any method on it and do not pass it around as an argument. If disposable object is passed to other methods (including calling its members, when "this" is implicitly passed), the tool should analyse if Dispose is not called within, or that object is not stored somewhere for later disposal.
That said, naive implementation of checking if disposable object is in fact disposed with "using" construct or in any other way would yield too much false positives, and render analysis useless.