Regression: Problems with Timestamp arguments

Hello colleagues,

There seems to be a problem with the latest driver and Timestamp
arguments uses in IS NULL comparisons:
QUERY: SELECT ? IS NULL
ARGUMENTS: statement.setTimestamp(1, new
Timestamp(System.currentTimeMillis()))

Result:
Exception in thread "main" org.postgresql.util.PSQLException: ERROR:
could not determine data type of parameter $1
at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2157)
at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:1886)
at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:255)
at org.postgresql.jdbc2.AbstractJdbc2Statement.execute(AbstractJdbc2Statement.java:555)
at org.postgresql.jdbc2.AbstractJdbc2Statement.executeWithFlags(AbstractJdbc2Statement.java:417)
at org.postgresql.jdbc2.AbstractJdbc2Statement.executeQuery(AbstractJdbc2Statement.java:302)

The problem arises on:
protocolVersion=3, or not specifying protocol version
argument type: Timestamp

The problem does not arise on other types (tested on BigDecimal and String)
The problem does not arise when using protocolVersion=2
The problem does not arise if the expression is not ? IS NULL

List of work-around methods:
- Degrade the protocol:
jdbc:postgresql://host:port/database?protocolVersion=2
* does not work with PgPool-2

- Explicitly cast the argument
SELECT ?::timestamp IS NULL
SELECT CAST(? AS timestamp) IS NULL
* requires rewriting of currently working code

I believe this to be a bug.