While hiking on a recent "June '08 Parade O'Fools" two-day jaunt in the Massachusetts Berkshire Mountains near CT, we encountered an hour plus of torrential downpour/lightening conditions during a late afternoon.
A couple of hour break in the storm allowed tarps and hammocks to be hung in relative dryness. Well, once tucked in at night, the lightening and dumping rain continued until 2:30 a.m.
Instead of bringing along an 8x10 tarp (even with the [this time accurate] forecast), decided to see how well my poncho would work as a tarp. Luckily, there was absolutely no leaking or wetness on the hammock. Also, luckily, there was no wind whipping the rain around.
If I had been hanging up in the NH White Mountains under similar conditions, I would most definitely have brought a real tarp, since the mountains typically create their own wind blowing downhill at night.
But in the case of heavy rain without heavy winds, a tarp seems to work just fine.