Invalid magic number in log file?

When starting up a postmaster from an older build on a database initialized
with a newer build I'm getting the following errors. I think I saw the same
thing earlier going in the opposite direction too. Shouldn't there be some
earlier errors firing from the control file version number?

stark@oxford:~$ /usr/local/pgsql/bin/postgres -D /var/tmp/db
LOG:  database system was shut down at 2007-05-14 19:47:51 BST
LOG:  invalid magic number D061 in log file 0, segment 0, offset 0
LOG:  invalid primary checkpoint record
LOG:  invalid magic number D061 in log file 0, segment 0, offset 0
LOG:  invalid secondary checkpoint record
PANIC:  could not locate a valid checkpoint record
LOG:  startup process (PID 9590) was terminated by signal 6: Aborted
LOG:  aborting startup due to startup process failure


--  Gregory Stark EnterpriseDB          http://www.enterprisedb.com