pg 8.1.2 ERROR: direct correlated subquery unsupported as initplan

Just encountered this error on an 8.1.2 cluster:

LOG: statement: PREPARE dbdpg_5 AS SELECT * FROM foo WHERE visitnum = $1 and (scheduledtime >= $2 OR scheduledtime IS NULL) AND (scheduledtime < $3 OR scheduledtime IS NULL) UNION SELECT * FROM bar WHERE visitnum = $4 ORDER BY scheduledtime
LOG: statement: <BIND>
LOG: statement: EXECUTE <unnamed> [PREPARE: SELECT * FROM foo WHERE visitnum = $1 and (scheduledtime >= $2 OR scheduledtime IS NULL) AND (scheduledtime < $3 OR scheduledtime IS NULL) UNION SELECT * FROM bar WHERE visitnum = $4 ORDER BY scheduledtime]
ERROR: direct correlated subquery unsupported as initplan

A run of 'analyze' seems to have made the error go away.

Here's our version:

PostgreSQL 8.1.2 on ia64-unknown-linux-gnu, compiled by GCC gcc (GCC) 3.4.3 20050227 (Red Hat 3.4.3-22.1)

Any clues?

Thanks,
Ed