ClassCastException DelegatingPreparedStatement

I'm just getting around to testing out the advice I got from this list a
couple weeks ago about prepare thresholds (thanks, Mark & Dave).

Using code modeled after the PG JDBC "Server Prepared Statements" code
example, I'm getting this runtime error:

java.lang.ClassCastException:
org.apache.tomcat.dbcp.dbcp.DelegatingPreparedStatement

on the line marked with an asterisk:

            try {
                Class.forName("org.postgresql.Driver");
            } catch(Exception e) {
                printError("org.postgresql.Driver class not found in
getMedlineListFromParsedText(): ", e.getMessage());
            }
            prestmt = conn.prepareStatement(query);
  *        org.postgresql.PGStatement pgstmt =
(org.postgresql.PGStatement)prestmt;
            pgstmt.setPrepareThreshold(0);

I'm using DBCP pooled connections within tomcat.  Versions: PG 8.1,
postgresql-8.1-407.jdbc3.jar, tomcat 5.5.20, Java 1.5

My Java newbie status is hobbling me.  Thanks for any pointers.

Thanks,
Kevin