About the only time I feel confident a 30 degree bag will do the trick is from about Father's Day to Labor Day.  And even at that, last year you'd have been cold some of the days in between there.

I've taken one trip so far with a Clark Hammock...loved it...and I found little difference in warmth between that and a tent.  One thing that helps is a thermarest of some sort underneath you.  Not only does it help spread the hammock a bit, it also adds a bit of warmth.