I'm using JDBC4 Postgresql Driver, Version 8.4-701.
Using pgJDBC, if you call clob.truncate(0) to truncate a clob to zero-length, you get a "org.postgresql.util.PSQLException: LOB positioning offsets start at 1". This is spurious -- the argument to truncate is not a position offset, its a length. So although position offsets must start at 1, lengths can start at 0. I see there is a call to assertPosition(len) -- that needs to go, maybe replace with a new assertLength(len)?
(I am not on list so please reply direct; I hope this gets through to you though...)