pg_locks.transaction field type

I have a method in my rmi server that takes a query and returns an
Object[][]. I had this query:

SELECT (select relname from pg_catalog.pg_class where relfilenode =
relation) as relname, * FROM pg_locks;

After upgrading from 7.4 to 8.0 I was getting this problem:
WARNING: Servlet.service() for servlet jsp threw exception
java.rmi.UnmarshalException: error unmarshalling return; nested
exception is:
java.lang.ClassNotFoundException: org.postgresql.util.PGobject
(no security manager: RMI class loader disabled)

The problem seems to be the "transaction" field. It looks like a number
type, but I can't really tell because "\d pg_catalog" doesn't work in psql.

So what's the difference between 7.4.7 and 8.0.3? Is it that 7.4.7
never returned anything in the transaction field? Is it a jdbc bug that
is returning the answer as org.postgresql.util.PGobject instead of some
kind of Number?