http://www.sitabus.it/sitabus/campania/orarioCAMPANIA/QUADROXV.pdf
Looking at the Summer 2010 SITA bus schedule for the Amalfi Coast, I note that the LAST bus from Sorrento to Positano/Amalfi; and from Amalfi to Sorrento (via Positano) depart at MIDNIGHT.
The last Amalfi to Ravello bus departs at 1:00 a.m.
The last Ravello to Amalfi bus departs at 0:25 a.m.
With these convenient schedules (which repeat those of last year), you'll be able to enjoy late evenings out for dinner or socializing, yet be able to get back to your hotel without paying for a taxi.