Spurious failure to obtain row lock possible in PG 9.1?

I'm using Postgres 9.1 on Debian Lenny and via a Java server (JBoss AS 6.1) I'm executing a simple "select ... for update" query:
SELECT
importing
FROM
customer
WHERE
id = :customer_id
FOR UPDATE NOWAIT
Once every 10 to 20 times Postgres fails to obtain the lock for no apparent reason:
18:22:18,285 WARN [org.hibernate.util.JDBCExceptionReporter] SQL Error: 0, SQLState: 55P0318:22:18,285 ERROR [org.hibernate.util.JDBCExceptionReporter] ERROR: could not obtain lock on row in relation "customer"
I'm "pretty" sure there's really no other process that has the lock, as I'm the only one on a test DB. If I execute the query immediately again, it does succeed in obtaining the lock. I can however not reproduce this via e.g. PGAdmin.

Is it possible or perhaps even known that PG has this behavior, or should I look for the cause in the Java code? (I'm using Java EE"s entity manager to execute a native query inside an EJB bean that lets a JDBC connection from a pool join a JTA transaction.)
Thanks!